Translation:Shulchan Aruch/Choshen Mishpat/3
There is no beit din less than three. And any three is called a beit din, even regular people (because it is impossible that there isn't among them one who knows judgment reasoning, but if there isn't one of them who knows, they are invalid from judging (Tur in the name of his father, the Rosh, beginning of Sanhedrin). But nevertheless, [the parties to the judgment] can accept the [possibility of] error and send them before a teacher) (Maharam Padua, section 43) and they can judge a person against his will if the defendant refuses to descend to judgment or if he doesn't want to have judgment with the prosecutor in his town; but if he does want to judge with him in his town, but just doesn't desire the three [judges] that the prosecutor selected, then [the prosecutor] chooses one [judge] and [the defendant] chooses one [judge]. Note: Like below in section 13. And it appears to me that this is specifically with unappointed judges, but if there are appointed judges in the city, he can't say "I won't be judged before them, but rather this [judge] I choose." And this is our practice in our town. And see later in section 22 paragraph 1.
Fewer than three, their judgment is not the law, even if they didn't err, unless the parties to judgment accepted him or he is an expert [to guide] the many. (And at this time, an expert [to guide] the many doesn't judge judgments, because he would judge alone against [the parties to the judgment]'s will) (Mahariv section 147). And whoever aren't three (and that [the parties to judgment] didn't accept upon themselves and isn't an expert [to guide] the many) (Tur), even if they were ordained in the land of Israel, the admission of someone who admits before them is like someone who admits outside of beit din, and they can change the thing they related before them. And someone who says-something(?) in front of them and afterwards witnesses came, he has no suspicion of having-said-that-thing(?); but [before] three, even though they weren't ordained, an admission before them is like an admission in beit din. And similarly, if someone says-something(?), and afterwards come witnesses, is suspected as having-said-that-thing(?) and he can't retract.
Even though an expert fit to guide the many is permitted to judge alone, it is a commandment of sages that others sit with him.
Even though a beit din of three is a full beit din, whenever they are many, they are praiseworthy. And it is better that they split the judgment with eleven than with 10, and who are sitting in the beit din need to be talmidei chachamim and fit, and it is forbidden for a smart person to sit at judgment until he knows with whom he will sit, lest he sit with unqualified people and it will turn out that he is in the category of helping traitors and not in the category of a beit din. Someone who isn't an expert and that the parties to the judgment didn't accept, even though he got permission from the Rosh HaGolah [to judge], his judgment isn't law, even if he didn't err; and every one of the parties to the judgment, if he wants, can return and be judged before a beit din. Note: The permission that an idolatrous king gives at this time isn't anything; however, if the congregation accepted him because of the letter of the king, he can judge (Tur). And there are those who say that if the judge is learned and wise, permission of the king (Rivash section 271) or the officer appointed in his town would take effect, and this is in the category of dina demalchuta to return judges who want; but nevertheless, someone who does this without permission of the congregation distresses the congregation, and will in the future be accountable in judgment (Responsa of Rashba, section 737).