R. Yonathan says: The first was stated in the beginning, and "beginnings are not expounded." "until elohim shall come the matter of both" — one. "Whom elohim shall incriminate" — two. And "there is no evenly balanced beth-din," wherefore one is added to them — so that we have three.
Rabbi Yonathan Tackled a Fundamental Question in Jewish Jurisprudence
Mekhilta Tractate Nezikin 15:14