Three people. A common noun indicating a group or count of three persons (usually read 'san-nin').
明日、三人で映画を見に行く予定です。
Tomorrow, the three of us plan to go see a movie.
クラスには三人の留学生がいる。
There are three exchange students in the class.
ことわざに『三人寄れば文殊の知恵』という言葉がある。
There is a proverb that says 'Three heads are better than one' (literally, 'If three people gather, the wisdom of Monju').