When you say three people ate like one, they seem to need to finish. Sheloshah she'achlu ke'echad (Three who ate together): echad mafsik (interrupts) be'al korcho (against his will) lishnayim, ve'oneh imahem birkat zimun (invitation to bless), and answers with them. The third person, who still wants to continue eating, is forced to stop and answer with them. He answers the zimun. If we stopped here, we would say that he is essentially being made to say Birkat Hamazon (Grace After Meals). The text continues: ve'afilu lo ratzah lehafsik, mezamnin alav, bein oneh bein eino oneh (even if he did not want to stop, they include him in the zimun whether he answers or not). That makes sense: you follow the majority, whether he wants to participate or not; he is part of it. Kol shehu omed sham (As long as he is there) does not mean that he has to be standing in this case.
But shnayim ein chayavim lehafsik le'echad, velikach ein chiyuv (obligation) zimun chal aleihem sheyitztarchu lehafsik le'echad (two are not obligated to stop for one, so the obligation of zimun does not fall on them in a way that requires them to stop for him). You follow the majority: if the two do not want to stop, they do not have to. Ve'im lo ratzu lehafsik vezimnu aleihem, lo asah kelum (if they did not want to stop and he included them in a zimun, he has done nothing). If he says the zimun by himself and the other two do not want to participate, it does not count.
The text continues: ve'im lo ratzu lehafsik, af hu eino rashai levarech velatzet lashuk ad sheyigmeru hashnayim veyezamen aleihem (if they did not want to stop, he is not permitted to bless and leave until the two finish and include him in the zimun). He is actually not allowed to say his own Birkat Hamazon at this point, because he has already become obligated in zimun. How can he bless without zimun?
It is interesting that the Shulchan Aruch does not tell us what happens to the first person in the first case. He has to stop Be'al korcho. But suppose he wants to continue eating: can he? If he continues eating, does he have to say a berachah (blessing) or not? Why does the Shulchan Aruch not discuss this? There is a major machloket rishonim (dispute among early authorities) about it. It would be useful to see the Beit Yosef. I do not like learning the Shulchan Aruch by itself, especially where there is a dispute among the early authorities. HaRav Ovadiah brings all those opinions, including Rav Hai Gaon, the Ra'avad, and the Rashba. We saw them in the Beit Yosef. The Ish Matzliach says that one should do this only if necessary, because otherwise you put the first person in a difficult position. The Bi'ur Halachah also brings the different opinions, but the Shulchan Aruch does not ask the question. Actually, he does ask it.
The Bi'ur Halachah says: vehineh beShulchan Aruch lo zachar kelal im meiri shehashnayim hayu nechutzim latzet, o be'az chadya be'al korcho ha'echad lehafsik lahem mise'udato. The wording there is unclear: “resh, siman resh”; then, “it’s not resh”; and later, “Siman resh.” These are serious doubts, so let us look at the Tur.
The Tur says: sheloshah she'achlu ke'echad, echad mafsik lishnayim ve'ein shnayim mafsikin le'echad (three ate together: one stops for two, but two do not stop for one). Rashi explains that it is derech eretz (proper conduct) for one to interrupt his meal for two, until they have said the blessing beginning Hazan et hakol (Who sustains all). That is, birkat hazimun includes the first berachah of Birkat Hamazon, through Hazan et hakol. The Rama will be posek halachah (decides halachah) this way, but the Tur is the source.
The Tur continues: vechozer vegomer se'udato (meal); he returns and finishes his meal. But two are not required to stop for one, unless they wish to do so lifnim mishurat hadin (beyond the letter of the law), to accommodate him. Rabbeinu Yonah says that one is obligated to stop for two. Rabbeinu Yonah does not say that stopping for two is merely derech eretz. He says it is an obligation.
The Ra'avad writes that one might initially understand that if the person did not stop, the others would not include him in the zimun. That is not so: they include him even though he intends to keep eating. Rather, he must stop so that he is yedei (fulfills) zimun with them, which he has already become obligated to do. The Tur adds that he stops until they finish Birkat Hazan. This is learned from the discussion, “Ad heichan birkat hazimun?”—how far does the zimun extend? One person wants to leave, and the question is how long he must remain while the others say the zimun.
Rav Nachman bar says that he must stay until “Nevarech she'achalnu mishelo uvetuvo chayinu”; that is the zimun, which requires three. Rav Sheshet says that he must stay until they finish Birkat Hazan, and the halachah follows Rav Sheshet. Just as an individual cannot leave before they finish Birkat Hazan, the same applies to one who stops for two.
Rav Hai writes that one who stops for two must say Hamotzi (Who brings forth bread) again. If the zimun includes Birkat Hazan, he has already said part of Birkat Hamazon; how can he return to eating without saying a berachah? The Ra'avad writes similarly: he must wash his hands again with netilat yadayim (washing of the hands) and break bread. They compare him to bnei chaburah (meal companions) who left to greet a groom; if nobody stayed, they need berachah lemafre'a (blessing retroactively). The Tur's father writes that the wording “interrupts” does not sound like that. It sounds as if he stops and then continues. This is not like meal companions who left: they got up and went away, while here he does not leave his place. They have only forced him to answer; there is no interruption of his attention from the meal. Rather, they continue and finish their meal without another berachah, as the Bahag also writes.
The Tur then asks his father about the opinion of the Ri, as explained in Tosafot and also by the Rosh: the individual who stops for two begins again at nodeh lecha (we thank You). If he has to wait until the end of Birkat Hazan, that means he has already said part of Birkat Hamazon. When he finishes his meal and says Birkat Hamazon on his own, where does he begin: at Hazan et hakol, or at nodeh lecha? The Ri says in Tosafot that he begins at nodeh lecha.
The Tur asks about three people who stopped for seven. Suppose ten people are eating: seven say Birkat Hamazon together, while three continue eating. How do those three later say a zimun? The answer is that they return to the beginning, as with the zimun “Nevarech she'achalnu mishelo,” because they joined only for “Eloheinu” (our God). As soon as they hear “Nevarech Eloheinu,” the three may eat. Therefore, they return to the beginning when they say their own zimun. But one who stops for two must wait until the end of Birkat Hazan. The first berachah is different in that case.
The explanation is that when you are part of a zimun of three, without you there would be no zimun, so you wait until Hazan et hakol. When you are part of a zimun of ten, they have a zimun without you, because there are three others; your participation is only for Eloheinu. You do not have to wait until they finish Hazan et hakol, and you are not yotzei of Hazan et hakol. You start again from the beginning. The Tur writes that he had already ruled that the person returns to the beginning of Birkat Hazan. From this, we learn that three who stop for seven need only stop until the others say “Nevarech Eloheinu.”
The Tur adds that he asked his brother, Rabbeinu Yechiel, whether someone who had stopped once could stop a second time, and his brother answered that he could. The speaker adds, “meaning if there's a few, I don't know exactly what the case is.”
The Beit Yosef brings the Gemara discussion, located with the words “bar ve-samuch, pa-pa-pam.” The Rosh also cites Rabbeinu Yonah but does not decide between Rashi’s explanation and Rabbeinu Yonah’s. The Rashba writes like Rabbeinu Yonah. That is an obligation. From the wording of the Shulchan Aruch, it sounds as though he rules that it is obligatory to stop, not that this is merely a matter of proper conduct. They also raised other objections.
The Beit Yosef also brings the Ra'avad’s explanation, which the Rosh cites, and the Rashba’s wording: “Echad mafsik lishnayim, shenayim ein mafsikin le'echad” (one stops for two; two do not stop for one). One person stops against his will for two and answers with them. Even if he does not want to stop, they include him whether or not he answers, as long as he is there. But two are not obligated to stop for one. Therefore, the obligation of zimun does not take effect until they agree to stop and bless.
If the two do not want to stop and he includes them in a zimun, he has done nothing. Therefore, if he does not want to stop, he is not permitted to bless and leave until the two finish and include him in the zimun. He has already become obligated in zimun, so how can he bless without zimun?
But the Beit Yosef says that he saw Rav Hai Gaon write that if one of them wants to leave, the other two do not interrupt their meal and say zimun with him. Rather, he blesses by himself, without zimun, and leaves, while they finish their meal. The Beit Yosef says that Rav Hai Gaon’s words require further examination.
Why would Rav Hai Gaon say that the person can leave? Is he patur (exempt) from the mitzvah (commandment) of zimun? If he says to the two guys, “I got to go. Do you want to do zimun now or should I say Birkat Hamazon?” the Rishonim say, “You don't have the choice. You started eating together, so you cannot leave; you have to do zimun first.” If he asks them to do zimun with him and they refuse, he is trapped. The dispute is about when the obligation of zimun comes into being: if they are not ready to do zimun and he cannot lock them in because he is by himself, then he is not obligated to wait, because the obligation comes when they are ready to do zimun.
In other words, there are two possible points of formation: when they get together to eat, or when they get together to say zimun. The analogy is a corporation: when was it formed, when they started eating or when they were ready to say Birkat Hamazon? There was also a possible point in the middle: how do you break it up? For Rav Hai Gaon, maybe eating together is a prerequisite, but it is not how the zimun is formed; the corporation is broken up before it started. For the other Rishonim, it has already started and cannot be stopped. That is Rav Hai Gaon’s position. Most of the Rishonim do not agree. They understand that once the three eat together, they are obligated. The zimun is not a new obligation; it is the end of the process that began when they ate together. The obligation does not begin when they are ready to say zimun; it begins when they decide to eat together. That is the dispute.
A participant says that it does not sit well with him and that he is not disagreeing with those authorities. A participant objects that they cannot force him to commit an averah (transgression) just because he has to leave. The dispute is precisely whether he has to wait. We will learn more about how this works in the Gemara.
Now, what is the zimun? Does it include only the berachah of zimun, or does it include the first berachah as well? The Beit Yosef says that the Tur’s explanation is from the Rosh, and not from Rashi. Rashi explains the question “Ad heichan birkat hazimun?” as asking how far the zimun extends—that is, how long three people must remain together, just as two cannot say the zimun. Rav Sheshet says that Birkat Hazan is also part of the zimun.
Tosafot and the Rosh ask against Rashi: we hold like Rav Sheshet in matters of prohibition, and an individual also says Birkat Hazan. We are accustomed to say Birkat Hazan even when alone. If Birkat Hazan is part of the zimun, why does an individual say it? Is it part of the zimun or not? The fact that an individual says it makes the question difficult. They also raised other objections.
Therefore, Tosafot and the Rosh explain that the question “Ad heichan birkat hazimun?” refers to the case of three people eating together, where one interrupts for two. He interrupts his meal to say the zimun and then returns to eating. Rav Nachman says that he stops until “Nevarech,” after which he begins eating again. Rav Sheshet says that he stops until Hazan, and Hazan is included. Nevarech itself is not a berachah. The reason, however, is not that Birkat Hazan is considered part of the zimun. Every individual says Hazan. Rather, it makes sense for him to pause his eating for one berachah, so it will be apparent that they are including him in the zimun. Otherwise, it would look as though two people are saying zimun while he continues eating as if nothing is happening.
They say only, “Nevarech she'achalnu mishelo, Baruch she'achalnu mishelo uvetuvo chayinu,” and then he continues eating. That alone may not make it apparent that he has stopped. Therefore, he waits through one berachah—not because Hazan is part of the zimun, but to make it recognizable that he is included. Tosafot and Rashi agree that he must stop until “Baruch atah Hashem, Hazan et hakol”; the question is why.
The Rosh also writes that the Yerushalmi says: “Sheloshah she'achlu ke'echad uvikesh echad meihem leilech”—three people ate together, and one asked to leave. At the house of Rav, they said that he should say the first berachah and leave. Which is the first berachah? They answered: birkat hazimun. Rav Zeira, in the name of Rabbi Yirmiyah, said: Hazan et hakol. So the Yerushalmi calls the zimun a berachah; Baruch she'achalnu is a berachah. The Rosh says that this can be explained as the case of one who stops for two: one person asked to leave, and the others stop for him. The question is how long he must wait.
The Beit Yosef finds a difficulty with this explanation. If the question refers only to one person stopping for two, it should not ask in general, “Ad heichan birkat hazimun?” The HaRif explains that the question is about someone who must leave: three people ate, and one needs to go, so he sits and waits while the others say the relevant berachah. Rav Nachman says he waits until “Nevarech she'achalnu mishelo uvetuvo chayinu,” which is the zimun. If there is an emergency, he could then say Birkat Hamazon in his car. Rav Sheshet says he waits until Hazan.
The HaRif rules like Rav Nachman, who says that he waits only until “Nevarech,” and that is what people do. Everyone says Birkat Hazan, which shows that Birkat Hazan is not included in the zimun. That is the difficulty with the Rosh’s wording: if it were part of the zimun, people would listen to it rather than say it themselves.
The Tur takes the HaRif’s explanation to be the main one: the dispute is about someone who needs to leave. But how does the HaRif know that, according to Rav Sheshet, the law for the person leaving and the person continuing to eat is the same? Although the HaRif rules like Rav Nachman, the Tur is concerned with Rav Sheshet’s view and rules like him in matters of prohibition. Tosafot and the Rosh also appear to rule like Rav Sheshet, because the halachah follows him in matters of prohibition. Rabbeinu Yonah also rules this way, as does the Bahag.
From what Tosafot and the Rosh write—that Rav Sheshet’s reason for saying “until Hazan” is not that Birkat Hazan is part of the zimun, since every individual says Hazan—it follows that the Tur’s wording is not ideal when he says that the person stops until they have said the zimun through Hazan. That wording makes it sound as though Hazan is part of the zimun. Tosafot explain that this is not the reason for waiting; he waits so they will say one berachah and it will be apparent that he is included. That was Rashi's view, which the commentators had already rejected.
The Tur was not ruling like Rashi on this point. He had to quote Rashi’s words and did not remove anything from them, but when he discussed “Ad heichan birkat hazimun,” he did not mention Rashi’s explanation. The Rambam appears to agree with the HaRif. In the second chapter, he writes that the order of Birkat Hamazon begins with Birkat Hazan, followed by Birkat Ha'aretz (the land). In the fifth chapter, he writes that when three people eat, they must say birkat zimun before Birkat Hamazon. That wording suggests that none of Birkat Hamazon is part of the zimun. The zimun is “Nevarech she'achalnu mishelo” alone. Since the HaRif and Rambam agree, the Beit Yosef says that this is the ruling to follow.
The Darkei Moshe immediately disagrees. Tosafot and the Rosh write that the halachah follows Rav Sheshet, and Rabbeinu Yonah writes that this is also the view of the Bahag. There are four major Rishonim in that direction. The Rama is not bound by the HaRif and Rambam, and therefore disagrees with the Beit Yosef. Ashkenazim wait until Hazan et hakol; Sefardim do not.
We still need to know what happens to the person who stopped and wants to continue eating. The ruling is that he finishes his meal and then begins Birkat Hazan, although he has already fulfilled the zimun for what he ate. At the beginning of the chapter, the question is, “Le'eichan hu chozer?”—where does he return? Rav Zevid says in the name of Rava that he returns to the beginning; the Sages say that he returns to the place where he stopped. The halachah is that he returns to the place where he stopped. Rashi explains: “Le'eichan hu chozer? Mishehein tzarich zimun,” and says this is according to Rav Sheshet concerning Birkat Hazan. He had to stop eating until the person leading the blessing finished Birkat Hazan, and when he returns to eating, the discussion is where he resumes.
This discussion applies to the Ashkenazi approach, not the Sefardi approach. Let us return to what Rav Hai wrote. The Rosh says: “In the chapter of Sheloshah she'achlu, I saw written in the name of Rabbeinu Hai that one who stops for two must say Hamotzi. The Ra'avad wrote similarly: since he interrupted, he has fulfilled the berachah retroactively and must wash his hands and break bread.” The expression “mafsik” suggests a hefsek (interruption), as if he were meal companions who had eaten and then stopped.
From the wording of Rav Hai and the Ra'avad, it appears that although he must say Hamotzi again, he does not have to say a berachah retroactively for what he ate. It is an interruption: he does not have to say Birkat Hamazon first and then Hamotzi again; he only has to say Hamotzi again. The interruption is considered as though he had blessed retroactively. The hefsek requires Hamotzi again, but when he eventually says Birkat Hamazon, it covers both portions of the meal. The question is: where is the Birkat Hamazon on the first part?
The Mordechai writes similarly in the name of Rabbeinu Chananel, and it appears from his words that the person does not need to bless retroactively for what he ate. Shibolei haLeket brings this view in the name of Rabbeinu Chananel and says that he needs to say only Hamotzi. But the Rashba comments on Rav Hai’s words that, since the person interrupted, an obligation of blessing has already taken effect. He learns this from the end of the meal: the person has become obligated, so he must bless for what he ate; if he wants to eat again, he must say Hamotzi for what he will eat. This makes sense because ve'achalta vesavata uverachta (You shall eat, be satisfied, and bless): they force him to bless because he is part of the zimun, even if he is not yet satisfied and only wants to eat more.
The Ra'avad also writes that, since the person interrupted, he has fulfilled the berachah retroactively and must wash his hands and break bread if he wants to eat more. He is like meal companions who ate and finished, and who need a berachah retroactively. “They need a berachah retroactively” means that they need to say Birkat Hamazon. When they return to eating, they need berachah lechatchilah (blessing in advance).
The Rashba gives another reason: meichal uvaruchei behadadei i efshar (eating and blessing simultaneously is impossible). It makes sense, according to that view, that two people are not forced to stop for one. Why would they be made to go through the whole process and then return to eating? But when two stop one person, the reasoning is that they ate together and began together, so they finish together. That means he has to say the zimun and Birkat Hamazon with them. It is difficult, but it makes sense according to this understanding of what zimun means.
The Beit Yosef notes that there is a wording error in what the Tur attributes to the Rosh. It should say that there is no interruption of the meal, but rather that he continues to a completely separate meal; that is how it appears in the Rosh’s rulings. Here we have major Rishonim: Rav Hai Gaon, Rabbeinu Chananel, the Ra'avad, the Rashba, and the Rosh. Rabbeinu Yonah’s view in this matter is like the Rosh: they return and finish their meal without another berachah. Rabbeinu Yonah adds that even if the person later decides not to eat at all, since at the time of Birkat Hamazon he intended to eat and did not intend for that blessing to fulfill his obligation, he does not need to say Birkat Hamazon again.
Since Rabbeinu Yonah, the Rosh, and the Bahag agree on one view, the Beit Yosef says that this is the ruling to follow. He also invokes sefek berachot derabbanan lehakel (leniency in rabbinic blessing doubts); he does not say Hamotzi. Shibolei haLeket writes that Rashi explains, “if the two finished their meal.” It appears from here that the one who stopped need not say Hamotzi again, since he has come to finish his meal. He also quotes: “Vechen horah haRam, bam, na'asot mah.” Orchot Chayim writes that Rabbeinu Meshulam responded to the Ra'avad’s view: “kevod haRa'avad bimkomo munach.” For practical halachah, the Beit Yosef again concludes that, because Rabbeinu Yonah, the Rosh, and the Bahag agree, and because of the doubt concerning rabbinic blessings, we follow that view.
This halachah is not a joke; as you said earlier, Rabbi Meir Mazuz said that it is very bedi'avad (after the fact). The only reason he does not say a berachah in all these cases is that there is a major safek (doubt): should he say Hamotzi again, netilat yadayim or Hamotzi again, and Birkat Hamazon again? That is the whole question. If there is a dispute about whether to say the berachah, you do not say it; but why put yourself in this situation? “Two people want to do zimun and one who doesn't...” “They put me into that situation.” It should be done only when they really have to leave and he really wants to continue eating; it is very much a bedi'avad case, not something to do quickly or casually.