Three, considered collectively; three in company or acting together; a set of three; three united.

A composition for three parts or three instruments.

The secondary, or episodical, movement of a minuet or scherzo, as in a sonata or symphony, or of a march, or of various dance forms; -- not limited to three parts or instruments.

(1) Music for three performers; in music that descends from Baroque practice, this implies two treble instruments and basso continuo. (2) The center section of form in the minuet-and-trio family, generally in somewhat reduced orchestration or more passive setting. This comes from the tradition of alternating full ensemble and reduced forces in evenings of dance. Have a look.