Gemara

(Mishnah - R. Meir): Monetary cases require three judges. Cases of theft and wounds require three.

2b - Question: Cases of theft or wounds are monetary cases! (Why are they listed separately?)

Answer #1 (R. Avahu): The Mishnah teaches monetary cases, then gives examples, i.e. theft and wounds. It omits admissions and loans.

Suggestion: Admissions and loans were omitted because they do not require three judges.

Rejection: R. Avahu taught that all agree that if two judged a monetary case, the verdict is invalid! Rather, admissions and loans were omitted because they do not require Mumchim (expert judges).

Regarding (a watchman suspected of) theft, it says "v'Nikrav... El ha'Elohim." (This means Mumchim, and it is written three times in the Parshah.)

Question: Does R. Avahu say that Eruv Parashiyos applies here (words in this Parashah teach about a different Parashah)?

If it does (every "ha'Elohim" written here really applies to loans), also admissions and loans should require Mumchim! If it does not, admissions and loans should not require three judges!

Answer: Really, Eruv Parashiyos applies here. R. Chanina explained why we do not require Mumchim:

R. Chanina: Mid'Oraisa, interrogation of witnesses for monetary and capital cases is the same - "Mishpat Echad Yihyeh Lachem". We enacted not to probe witnesses in monetary cases, lest this discourage loans. (If the witnesses will err, they are disqualified and the lender will not collect. Similarly, the need for Mumchim makes it hard for lenders to collect.)

Objection #1 (against Answer #1): If monetary cases, i.e. theft and wounds require Mumchim, but admissions and loans do not, how can R. Avahu say that theft and wounds are examples of monetary cases!)

Objection #2: (If theft and wounds are examples of monetary cases) why does it say 'three judges' regarding monetary cases, and again regarding theft and wounds?

Answer #2 (Rava): (Eruv Parashiyos applies here. Mid'Oraisa, three Mumchim are needed also for admissions and loans.) The Mishnah teaches two different laws. (Mid'Rabanan, three laymen suffice for monetary cases, i.e. admissions and loans, lest people be afraid to lend. This does not apply to theft and wounds, so they require three Mumchim.)

(Rav Acha brei d'Rav Ika): Mid'Oraisa, one judge (even a layman) suffices for admissions and loans - "b'Tzedek Tishpot Amisecha". (Rashi - Eruv Parashiyos does not apply here. Tosfos - the verse allows one judge.) Chachamim require three judges, lest an ignoramus give the wrong verdict.

(Practically,) Rava and Rav Acha argue about Shmuel's law.

Shmuel: If two judged a monetary case, the verdict stands, just it is called an impudent Beis Din.

Rava argues with Shmuel, and Rav Acha agrees with Shmuel.

5a: Once, Mar Zutra brei d'Rav Nachman judged a case and erred. Rav Yosef said that he is exempt only if the parties accepted him for a judge.

(Rav and Shmuel): If one wants to judge and be exempt if he errs, he should ask Reshus (permission) from the Reish Galusa.

Rishonim

The Rif (Sanhedrin 1a) brings R. Avahu's answer, which relies on R. Chanina (we allow laymen, lest lenders be deterred).

Rif (ibid.): We learn from the case of Mar Zutra that the verdict of a lone Mumcheh stands even if he did not have Reshus.

Rosh (Sanhedrin 1:2): This is like the opinion that Eruv Parashiyos does not apply here. Mid'Oraisa, one judge, even a layman, suffices. Chachamim enacted that three should judge. Regarding a Mumcheh, they leave the law like mid'Oraisa. Regarding a layman, l'Chatchilah he must judge amidst three. B'Di'eved, if he judged alone it is valid. According to the opinion that Eruv Parashiyos applies here, the Torah requires three Mumchim. Chachamim enacted that three laymen or one Mumcheh suffices. If one or two laymen judged, b'Di'eved it is invalid. Mumcheh l'Rabim means one who has learned and can reason. Rav Shrirah Gaon says that it is someone like Rav Nachman in his generation. He must be fluent in Mishnah, Gemara, discretion, has investigated Dinim for many years, has been tested many times and has not erred. Such a judge can judge a person against his will.

Rambam (Hilchos Sanhedrin 2:10): Even though Beis Din is not less than three, "b'Tzedek Tishpot Amisecha" allows one to judge alone. Mid'Rabanan three are required. If two people judged, the verdict is invalid.

Rambam (11): If one is Mumcheh l'Rabim, or he has Reshus from Beis Din, he may judge alone, but he is not considered a Beis Din. Even though it is permitted, it is a Mitzvah mid'Rabanan to seat other judges with himself. Chachamim admonished 'do not judge alone', for there is only One lone Judge.

Rambam (5:8): Cases of fines such as theft, wounds, payment of double (for theft) and four or five times (for slaughter or sale of a stolen animal), rape, enticement, etc. require three Mumchim who received Semichah in Eretz Yisrael. Other monetary cases such as admissions and loans do not require Mumchim; even three laymen or even one Mumcheh may judge them. Therefore we judge admissions and loans in Chutz la'Aretz. Even though Beis Din of Chutz la'Aretz is not called 'Elohim', they are Sheluchim of the Beis Din in Eretz Yisrael. Their Shelichus does not extend to fines.

Rosh (Sanhedrin 1:1): Rava explains that even admissions and loans should require three Mumchim; we expound Eruv Parashiyos. However, R. Chanina taught that we enacted not to probe witnesses in monetary cases, lest this discourage loans. For the same reason we do not require Mumchim for admissions and loans. Similarly, for Kesuvos, inheritance and gifts, we do not require Mumchim or interrogation, to prevent injustice, i.e. lest one act unfairly, hoping that his victim will be unable to find Mumchim, or that the witnesses will not pass interrogation. Theft and wounds require Mumchim and interrogation.

Rosh (ibid): Rav Acha brei d'Rav Ika says that even though Eruv Parashiyos applies here, "b'Tzedek Tishpot Amisecha" teaches that Eruv Parashiyos does not apply to judges. Even one judge, even a layman, suffices. Mid'Rabanan, we require three judges, lest an ignoramus give the wrong verdict. Surely, at least one of the three has learned. If none have learned, the Beis Din is Pasul. We learn from Daf 23a; each party can disqualify judges picked by the other party, i.e. if they are unlearned.

Poskim

Shulchan Aruch (CM 3:2): If less than three people judged, the verdict is invalid unless the parties accepted them, or if the judge was Mumcheh l'Rabim. This is even if they did not err.

Gra (9): The Stam Gemara in many places is like R. Avahu. Also Rava holds like him, and qqu and Reish Lakish in the Yerushalmi.

Shulchan Aruch ibid.): If they are less than three (Rema - and the parties did not accept them, and they are not Mumchim l'Rabim), even if they received Semichah in Eretz Yisrael, admission in front of them is (invalid) like admission outside of Beis Din. The parties can make claims (when they later go to Beis Din) unlike their claims in front of the first judges. If one denied money in front of them and was refuted by witnesses, he is not Huchzak Kafran (established to be a denier). If there are three judges, even if they did not receive Semichah, admission in front of them is a (proper) admission in Beis Din. If one denied money in front of them and was refuted by witnesses, he is Huchzak Kafran. The parties cannot change claims they made in front of them.

Gra (9): Even Shmuel agrees that Odisa (admission) and validation of documents require three judges.