Gemara

(Rav): If two brothers divided an estate, and a third brother came from abroad, the division is void;

(Shmuel): Each brother gives from his share to the third brother.

Question (Rava): According to Rav, the division is void. This means that a division without the knowledge of one of the parties is invalid. If so, if there were three partners or brothers, and two of them divided by themselves, will we say that the division is invalid?!

Answer (Rav Nachman): There, they picked portions knowing that there were three. In Rav's case, they thought that there were only two.

(Rav): If brothers divided, and a creditor (of their father) took the portion of Ploni (a brother), the division is void.

(Shmuel): Ploni lost. He is not compensated.

(Rav Asi): Ploni gets a quarter of what was taken, in land v'Kesf (and/or money).

Rav says that the division is void. It as if each brother inherited his proper share; all must pay the debt. Shmuel says that Ploni lost. It is as if the brothers are buyers (each takes a share in place of his 'destined' inheritance) without Acharayos. Rav Asi is unsure whether they are like heirs or like buyers, therefore we compromise and he gets a quarter in land and/or money.

(Rav Papa): The Halachah is, in both arguments, the brothers give a portion of their property. (They are heirs, like Rav. The division is valid, like Shmuel);

(Ameimar): The division is invalid (like Rav).

The Halachah follows Ameimar.

Bava Metzia 31b: Rav Safra and Isur had a joint business venture. Rav Safra, in front of two people, took half the merchandise without telling Isur. Isur came to Rabah bar Rav Huna (to complain).

Rabah bar Rav Huna (to Rav Safra): Bring two of the three people in front of whom you divided, or two witnesses who saw you divide in front of three.

69a: Two Kusim made an Iska together. One divided the money by himself. Rav Papa ruled that the division stands, for Rav Nachman said that money is as if it was already divided. It does not need an evaluation. We do not say so about wine (different wines are of different qualities), or when there were good and bad coins.

Kesuvos 100a (Shmuel): If orphans come to divide their father's property, Beis Din appoints an overseer for them and they select a nice portion for them. When the orphans grow up, they can demand a new division;

(Rav Nachman): They cannot demand a new division. If they could, Beis Din would be weak!

The case is, Beis Din erred about location (e.g. an orphan received a portion far from property that he inherited from his mother's father).

Rishonim

Rif and Rosh (7:2): We say that if two out of three divided by themselves, the division stands. We learn from here that if one of two partners or brothers went away, and the other needs his share, we divide and give to him his share. We do not wait for his partner to return.

Rosh: This is like Rabah bar Rav Huna ruled in the case of Rav Safra and Isur, and the case of the two Kusim. Here we learn that the same applies to land.

Question (Rosh): In Kesuvos, we say that when orphans come to divide, Beis Din appoints an Apotropos and they pick good shares; if not for the power of Beis Din, we would think that when they mature, they can protest about which directions they received, even though they divided through a lottery. Here, there is no reason why the power of Beis Din should stop one from protesting when they divided in his absence!

Answer #1 (Rosh): There is different, for all the orphans are minors, and they do not demand a division. If one was an adult and demanded a division, we would not need the power of Beis Din.

Answer #2 (Rosh citing R. Yonah): There, if not for Beis Din's power, when they mature, one (David) could say 'my brother's portion is worth more to me.' Letter of the law, the division is Batel, for if before dividing he said so and the others said that it is not worth more to them, David would get it without a lottery. If a brother merely wanted a new lottery, hoping to get a different portion, surely we do not heed him. Likewise, here the third cannot protest against the division of the other two. Since the division was done through Beis Din and a lottery, what difference would it make if he were there?! However, if he says that another portion is worth more to him and he is willing to accept it for more, we heed him, for had he said so before the lottery, he would get it without a lottery. Also the Ra'avad said so.

Rif and Hagahos Ashri (Bava Kama 26a, 6:16): The Yerushalmi cites a case in which Bar Ziza's sharecropper deposited a Litra of gold with Ploni. Bar Ziza and his sharecropper died.R. Yishmael b'Rebbi Yosi said that surely, the gold was Bar Ziza's. Bar Ziza had minor and adult children. R. Yishmael gave half the gold to the adults, and said that the minors will get the other half when they mature. After R. Yishmael b'Rebbi Yosi died, the case came to R. Chiya. He ruled that the money Ziza's. Rather, it is given to the sharecropper's children. When he heard that Bar Ziza's adult children already received half, he said that they keep it, for it was through Beis Din, but the rest goes to the sharecropper's children. Bar Ziza's minors do not get a share of what their older brothers received.

Rif: What their older brothers received was like a Metzi'ah. R. Yitzchak says that Leis Din (there is no case) between the minors and adults. It is as if he received a gift.

Yam Shel Shlomo (Bava Kama 6:32): The minors have no case against the adults. It is as if they received a gift or found a Metzi'ah.

Rebuttal (Shach CM 175:2): R. Yitzchak argues with the first opinion. He holds that it is as if Bar Ziza received a gift, and the minors share it. Hagahos Ashri could agree, but he rules like the first opinion. Since it is an argument in the Yerushalmi, the older brothers are Muchzak, so they need not give. If so, if the minors seized (up to half) in a way that helps, they keep it. All old texts of the Rif say Leis Bein (there is no difference) between the minors and adults, i.e. they share equally what the adults received. This is the text in the Yerushalmi, in Hagahos Maimoniyos (Hilchos Nizkei Mamon 14 [5]) and Ba'al ha'Itur. If R. Yitzchak wanted to teach that the minors do not get, what is his Chidush? Also, he should have said just 'it is as if they got a gift.' Why did he say 'Leis Bein...'? Also, the Rif says 'it is as if he (Bar Ziza) received a gift. Indeed, the Yerushalmi says 'they', but this can refer to his heirs, unlike a Metzi'ah, which is only the finder's. Therefore, if a judge ruled and gave to the adults, and afterwards another judge ruled oppositely, it is a Safek whether the adults must give to the minors, so if they seized, it helps.

Rambam (Hilchos Nachalos 10:1): If two brothers divided, and a third brother came from abroad, or three brothers divided and a creditor took the portion of one of them, even if one took land and the other money, the division is void. They divide the rest equally.

Poskim

Shulchan Aruch (CM 175:3): If two brothers divided, and later came a brother that they did not know about, the division is void. Even if there were three fields and each took one and they split the third, and when the new brother came he received that third field, the first brothers do not keep what they received the first time. Rather, if one wants, he can Mevatel the old division and they make a new lottery for all three of them.

Beis Yosef (DH v'Im): The Rashbam explains that they initially divided in front of a Beis Din of commoners.

Beis Yosef (DH v'Da): Tosfos says that Shmuel says that each of the first two brothers gives a third of his share to the third brother, i.e. when initially each took one field, and they left one field for the third, in case his lottery will fall on it when he comes. Rashi said that Shmuel holds that the initial division stands even for the third brother, even though they divided without his Da'as. The Rosh and Tur were not concerned for Tosfos' opinion, for they challenged Rashi from the case of orphans, and answered that there is different, for no one requested to divide.

Gra (10): The Shulchan Aruch rules like Tosfos. Since Shmuel says that Shmuel upholds the division only when the third brother received the third field, Rav holds that even in this case the division is Batel.

Shulchan Aruch (ibid): Even if the third brother agreed to take part of the portions of the other two, either of the first can Mevatel the division, since it was mistaken.

Beis Yosef (DH v'Chosav Rabeinu): If the first brothers want to keep the initial division, surely Beis Din does not Mevatel it!