Sanhedrin 13 - Semichah of the Sages
If the majority of the Jews transgressed and worshiped an idol, because the Sanhedrin mistakenly ruled that this was permitted, and then they realized their mistake, the Sages of that Sanhedrin bring a bull, lean their hands on it, and confess. This is called Semichah, and three Sages are required for that.
Semichah can also mean Rabbinical ordination in an unbroken chain since Moses. Once the Romans enacted that anyone who confers ordination be killed, anyone who accepts ordination be killed, and the nearest town be destroyed. Yehudah ben Bava placed himself between two mountains and ordained five Sages there. When the enemies discovered them, he said to his students, "My sons, run! - and I will be like a rock that cannot be turned." The soldiers drove three hundred spears through him and made him like a sieve. Three Sages are required for ordination, but Yehudah ben Bava alone is mentioned because of his importance.
Art: Jean-Honore Fragonard - Jeroboam Offering Sacrifice for the Idol