Chabad.org — English translation + Rashi
Torah Temimah points to Sanhedrin 13b (unit 1; single primary locus per emergency Core plan).
E-Daf.com — Sanhedrin 13b
HebrewBooks / Shas PDF
English: It was taught: Rabbi Shimon says: The elders’ semichah is with three; why? “Elders” implies two, and a beit din is not even-numbered, so they add one more.
Minimum of three for elders’ semichah — at least three members of the Sanhedrin perform semichah on the communal bull.
Semichah on the par he’elam davar of the congregation requires a minimum of three elders. The plural “ziknei” supplies two, and the general rule that a beit din may not be even-numbered adds a third. Fewer than three does not constitute a valid performance of the elders’ leaning.