Sanhedrin 3 - Dissenting Views on the Number of Judges
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