Rashi on Deuteronomy 17:6/רש"י על דברים י״ז:ו׳

1שנים עדים או שלשה [BY THE MOUTH OF] TWO WITNESSES OR THREE [WITNESSES, SHALL HE THAT IS WORTHY OF DEATH BE PUT TO DEATH] — But if evidence can be established by two witnesses to what end does it (Scripture) mention to you explicitly that it may be established by three? Scripture does so in order to compare the evidence of three witnesses to that of two (to make the same law apply to both cases). How is it in the case of two? Their evidence forms one testimony; so, too, does the evidence of three (or many) witnesses form one testimony and they cannot be declared “plotting witnesses” unless all of them are proved to be “plotting witnesses” (Makkot 5b).