Humor uses threes as the completion to a joke. "The Rule of Three" is part of the formula. Many stories have you heard start out, "There was a priest, a minister and a rabbi … ." Three characters seems to work well when you are populating your own stories to make them funnier.
Several centuries ago, the Pope declared that all Jews had to leave Vatican City. Naturally, a great uproar ensued in the Jewish community. So, the Pope made a deal. He would have a religious debate with a representative of the Jewish community. If the Jewish representative won the debate, the Jews could stay. If the Pope won the debate, the Jews had to leave.