Sanhedrin 5a - Minimum Amount of Judges

The gemara quotes a braisa that says that monetary issues must be judged with 3, but a mumcha l'rabim (expert) can judge alone. Both R. Nachman and R. Chiya considered themselves worthy of judging alone. Rashi holds that this braisa is proof to R. Acha Brei d'rav ika on 3a who learns from the pasuk בצדק תשפוט עמיתך that m'doraysa one judge is sufficient. The gemara holds that the statement of Shmuel quoted on 3a who says that 2 judges can judge, just that they are considered chatzuf is based on R. Acha. Tosafos points out that the braisa would then be a contradiction to the opinion of R. Avahu 2b who says that 2 judges are worthless because m'doraysa we require 3 judges. Tosafos explains that even R. Avahu can agree with the braisa that one judge works because even if m'doraysa we require 3 experts (which is the opinion of R. Avahu), the Rabbonon allow for a mumcheh l'rabim to judge alone.

Regarding the forcing of the litigants to appear before the court, Tosafos 5a d.h. dan, explains that the 3 hedyotos, or yachid mumcheh l'rabim can force the litigants to appear before them, because in a case where the litigants accept him, he wouldn't need to be a mumcheh l'rabim. However, Tosafos explains that if the litigant is willing to go to court just that he wants a say about who the judge will be, we allow each one to pick a judge and agree on a third. But, if he is not willing to go to any beis din in the city, any beis din can force him to appear before them. This is paskened in Shulchan Aruch (c.m. 3:1) that the defendant has the right to pick a judge and agree on a third, but if he refuses all courts in the city of the claimant, he can be forced to appear before them. The Rama adds that the right of the defendant to pick a judge only applies when there is no established beis din in the city, but if there is an established beis din, they can force the defendant to appear before them. Ketzos HaChoshen (3:1) explains based on tosafos 2b that even if an individual who is not a mumche can judge alone, he cannot force them to appear before him unless he is a mumcheh.

R. Akiva Eiger (on shulchan aruch 3:1) quotes 2 interesting points: 1. If the minhag in a certain place is to follow a system of law designed by the merchants, one can use that system to make a claim against the other and the defendant cannot insist on being judged according to din torah. In these type of situations we apply the rule of minhag overriding the halacha. 2. If the defendant was acquitted in one court, the claimant cannot summon him to court again, and it is forbidden for the beis din to even hear the claim. Furthermore, the defendant is believed to say that he has already been acquitted by another court, unless the claimant can provide witnesses that he is lying.