The Beraisa explicitly says that three people are called a Shayara, and therefore when the Mishnah says "Shayara" it must also mean three people.

[Why is Shayara a term used for three people? It seems to me that the simple reason is because while one person is referred to in the singular and two in the plural, three is one above plural. Accordingly, when giving a unique name to a group of people, it must be there are at least three people in the group. Otherwise, one would simply be called a person, and two would be either "people" (as we know the rule is that "Mi'ut Rabim Shenayim" -- "the smallest plural is assumed to be two"). or "two." This could be why the Yerushalmi in Eruvin here and the Yad Malachi (see Klalim 578-580) say that there are a few names of "groups" that mean a minimum of three people.]

All the best,

Yaakov Montrose