Rav Acha says
that according to Biblical law, one judge is qualified to adjudicate
loan cases, because “...with
justice shall thou judge your fellow” is expressed in singular form
“thou.” However, because of the concern that a solitary judge might be
chosen from commoners, not knowledgeable in monetary law, the Sages
decreed that three judges are necessary even in loan cases.
But could three judges not be all commoners, not knowledgeable in
monetary law? - No, at least one of them will have picked some
knowledge. If so, they should not be liable for errors, like
professional judges? Were they to be exempt, they would be even more
ignorant corner dwellers.
In the verses of “...the
master of the house shall approach a judge...” the word “judge” is
mentioned three times, so three judges are needed. However, Rebbi says
that five judges are needed to start, so that at least three reach the
same conclusion.
Art: Barthel Bruyn - Portrait of a Man with Three Sons