מדרש על סנהדרין 68:7

"and by their word shall every contention and every plague-spot be (ruled upon."): Contentions are being compared to plague-spots, and plague-spots to contentions. Just as plague-spots are (ruled upon) only in the daytime, so, contentions. And just as contentions are not (ruled upon) by kin, so, plague-spots. If so, why not say: Just as contentions require three (judges), so, plague-spots.