According to the Mishna, among the ceremonies that require the participation of three judges is semikhat zekenim – rabbinic ordination.
In searching for a source for this requirement, Abaye points out a difficulty – if the source is the passage (Bamidbar 27:23) where Moshe lays his hands on Yehoshua to declare him his successor, then it would appear that a single judge would suffice. And if we saw that Moshe embodies the Sanhedrin and is considered the equivalent of its 71 members, then we should need a full Sanhedrin to confer rabbinic ordination.
Although the Gemara does not offer a response to that question, some suggest that there must have been others standing together with Moshe who are not mentioned because of the great honor given to Moshe. This answer is based on the continuation of the Gemara that describes a unique situation of semikhat zekenim.
Rav Yehuda quotes Rav as telling about Rabbi Yehuda ben Bava who must be remembered for keeping the laws of kenasot – penalties – from being forgotten. The Gemara explains that under Hadrian the Roman government forbade the conferring of rabbinic ordination. They announced that anyone giving or receiving ordination would be killed and nearby cities and provinces would be destroyed and uprooted. Rabbi Yehuda ben Bava gathered five students to a place midway between large cities and mountains and conferred rabbinic ordination on Rabbi Meir, Rabbi Yehuda, Rabbi Shimon, Rabbi Yosei and Rabbi Elazar ben Shammua. While he was killed for his efforts, his students survived to act as teachers and judges.
In this case the Gemara explains that he had other partners in this act, but they are not mentioned due to the honor that they wanted to give to Rabbi Yehuda ben Bava. Many rishonim believe that only one of the three people giving semikha needs to have ordination himself, in which case Rabbi Yehuda ben Bava did not need other people as he could have had his students participate in granting ordination to their friends, and then have them switch positions to confer ordination on the rest of the group.