Gemara

5b (Rav Nachman): If two judged a monetary case, the verdict stands, just it is called an impudent Beis Din.

Question (Rava - Mishnah): Even if two judges agree, and the third says 'I do not know', we must add more judges. (It is as if the third is not there).

According to Rav Nachman, the judgment of two is valid!

Answer: There, because they intended to judge in a Beis Din of three, they must give a verdict with three judges.

6b (Beraisa - R. Yehoshua ben Lakish): If two come for Din, and one is tough, you may withdraw (if you fear reprisal if you obligate him) until you know which way the verdict leans, but not after this - "Lo Saguru Mipnei Ish."

17a (R. Avahu): When we add judges (because one judge could not decide), we can make a Beis Din with an even number of judges l'Chatchilah.

Question: This is obvious! (An even number of judges decided, and we add two more.)

Answer: One might have thought that we count the indecisive judge, and if he later gives a reason, we heed him. R. Avahu teaches that this is not so. It is as if he is not, and we do not heed him, even if he gives a reason.

30a: If two exempted and one was Mechayev, and later we find that the two erred, they compensate the party they caused to lose, but they do not pay the share of the third judge. They could not have judged without him!

Avodah Zarah 72a: If one said 'I sell this to you for what three will estimate it', if two of the three agree, that is the price. (They are like Beis Din.) If he said 'as three will say', all three must agree on the price.

If he said 'as four estimate it', all four must agree (he does not want a Beis Din, rather, many opinions), and all the more so if he said 'as four will say.'

Rishonim

Rambam (Hilchos Sanhedrin 8:2): If in a Beis Din of three, one judge is unsure, whether the others agree or argue, we add two judges. The five debate... If at least four decide, we follow the majority, even if now the same or a different judge is undecided. If the judges were half-half and one is unsure, we add two. We keep adding like this until 71. If 35 acquit and 35 convict and one is unsure, they debate with him until he decides. If he does not decide and none of others retracts, the Din is a Safek; we leave the money in its Chazakah.

Rebuttal (Ra'avad): The Gemara says that the undecided judge is not counted. After we add, there are only four judges.

Hagahos Ashri (1:6): If three sat to judge and one removed himself before he knew how the Din leans, the other two cannot finish the Din. They are no better than two who agree and the third is undecided. We must add judges, since initially three sat to judge.

Beis Yosef (CM 12 DH mi'Divrei): This shows that if three sat to judge, one may remove himself only before he knows how the Din leans.

Rebuttal (Bach DH Beis Yosef and Darchei Moshe 2): Hagahos Ashri said only that if he removed himself before he knew how the Din leans, the other cannot finish. If he knew how the Din leans, the other two may finish it. If one said 'as three will estimate it', it suffices if two of the three agree (Avodah Zarah 72a). Perhaps one may withdraw after he knows how the Din leans, and the other two will finish the Din, since he does not withdrew due to fear of a litigant.

Rashba (2:104, brought in Beis Yosef CM 13 DH Kosav ha'Rashba): A case occurred in which the parties chose 10 people to judge them for Din or Pesharah (compromise), and if they do not agree they will follow the majority, like Torah law. One of the 10 withdrew, and does not want to be part of this or give his opinion. Clearly, if they were chosen for Din and one withdrew, or says that he is unsure, the remaining nine cannot give a verdict. We follow only a majority of those who acquit over those who convict, or vice-versa. If the minority is not here, we do not follow the majority. Perhaps if he were here, he would give a reason unlike the majority, and they would agree! The same applies when one is unsure. When one acquits and one convicts and one is unsure, we add judges. The other two cannot decide by themselves. We say (30a) that if a judge (Ploni) dissented and the verdict was given and it was found that it was a mistake, the erring judges do not pay Ploni's share, for they could not have judged the case without him. The same applies to Pesharah. Why should it be different?!

Poskim

Shulchan Aruch (CM 13:7): If the parties chose 10 people to judge them, whether for Din or Pesharah, and if they do not agree they will follow the majority, and one withdrew and does not give his opinion or says that he is unsure, even if the other nine agree, this means nothing (there is no verdict).

SMA (20 and Gra 17): If they did not stipulate, we do not follow the majority for Pesharah (CM 12:18, from Avodah Zarah 72a).

Shulchan Aruch (18:1): If in a Beis Din of three, one judge is unsure, we add two judges. If one of the five is unsure, if there is a majority among the other four, we follow it.

Rema: This seems to me primary. The Shulchan Aruch wrote oppositely in 13:7. That is the dissenting opinion. Perhaps the Shulchan Aruch distinguishes between whether more than three were chosen from the beginning, or if they began with three. I disagree. Rather, the first is the Rambam's opinion, and the latter is the Rashba's.

Rebuttal (Bach DH Kosav ha'Rav): The Rambam agrees that if 10 were picked to rule a case, if one is unsure, the other nine cannot rule. The Rambam says that we ignore the undecided judge only when they three sat to judge, and one of them was undecided, and we add two, for then we intended to finish only with four. The Rashba only argues about whether the undecided judge may debate with the other four. He agrees that all of the four must decide. He said that we follow the majority only amidst debate, lest one ask 'since three suffices for Beis Din, why do we care about the undecided one?' However, why does the Gemara say 'it is as if he is not, and we do not heed him, even if he gives a reason'? Also the undecided had reasons to acquit or convict, like it says, 'if he gives a reason...'! If so, our decision was amidst a debate among all 10! Rather, the Rashba discusses one who withdrew, but (also) if he had reasons but did not decide, we do not finish the Din with others, and we do not follow the majority. However, the Kesef Mishneh cites the Rashba to say that if we added two and one of them is unsure, we follow the majority of the three who decide. This is like the first opinion in the Yerushalmi; the Rambam rules like the latter opinion.

Gra (8): The Rema is difficult, for the Rashba holds that we can have a verdict if (we added two and) two acquit, one convicts, and two are unsure! However, the Rema's distinction is correct. If two judged a monetary case, the verdict stands, but if they intended to judge in a Beis Din of three, they must give a verdict with three (5b), and similarly when they intended to judge in a Beis Din of four or more.

Ketzos ha'Choshen (1): The Ran (5b) says that when they intended to judge in a Beis Din of three, the verdict requires three, for they do not want to be called an impudent Beis Din. This does not apply to 10!

Shulchan Aruch (4): If three sat to judge and one removed himself, the other two cannot finish the Din.

Rema: The same applies if more than three were initially chosen, and one removed himself, like it says in Siman 13:7.

SMA (11): The Shulchan Aruch (13:7) equates one who is undecided with one who removed himself. This is based on Teshuvas ha'Rashba. The Rema distinguishes them. He argues (Sa'if 1) about when one judge is undecided, but he agrees here when one removed himself. Perhaps this is because if the 10th judge were here, perhaps he would give a reason and persuade the others. However, when a judge is undecided, he did not find a reason. Then we follow the majority, for it includes the three required for Beis Din.