Musar for Sanhedrin 45:5
הכי קאמר כשזה בורר לו ב"ד אחד וזה בורר לו ב"ד אחד שניהן בוררין להן עוד אחד
<b><i>GEMARA</i></b>. Why should each of the parties choose one [Bethdin]:<span class="x" onmousemove="('comment',' Which consists of three judges. By 'ONE' in the Mishnah, the text understands a court, according to which interpretation nine judges are necessary. So Rashi. This, however, is a very strained interpretation, particularly in view of the opening statement of the Mishnah: CIVIL ACTIONS ARE TO BE TRIED BY THREE. Tosaf. therefore states that the question is based on the assumption that the meaning of the Mishnah is this: Each litigant chooses a complete Beth din; and then the two courts jointly nominate a third court, and it is the third court that tries the case. Hence the question: Why such a clumsy proceeding: cannot the two litigants jointly select one court which shall try the action? ');"><sup>3</sup></span>