The Babylonian Talmud: Tract Sanhedrin

GEMARA: How is to be understood the expression of the Mishna: One party selects one, etc.? Does it mean one party may select one court of three judges, and likewise the other; and then both the third court, which would be altogether nine judges? Are, then, three not sufficient? It means, if one party selects one judge its opponent may also do so, and then both may select the third one. And what is the reason of such a selection? It was said in Palestine in the name of R. Zera: Because each party selects its own judge, and both agree in the selection of the third one, the decision will be a just one.