TT #228 · Parashas Shoftim · Sixth Aliyah · Passuk 19:15
A single witness shall not rise up against a man for any iniquity or for any sin, in any sin that he may commit; by the mouth of two witnesses or by the mouth of three witnesses a matter shall be established.
Torah Temimah points to Sanhedrin 30a–b.
TT lemma: אוֹ עַל פִּי שְׁלֹשָׁה עֵדִים.
English: ‘Or by the mouth of three witnesses’ – to equate three with two.
Derash equating three with two (Bavli) — the verse places three on the same footing as two.
A group of three witnesses is governed by the same principles that govern a pair; the extra witness does not create a different category.