Generally speaking, a trio or threesome is a group of three.

The word is most often used in music to refer to a group of three musicians. For example, a group of three singers would be called a trio.

The word is also used in chamber music to mean a grouping of three instrumentalists or any piece written for such a grouping. The most common forms of trio in classical music are the piano trio (piano, violin and cello) and the string trio (violin, viola and cello).

The trio is also the third section of a march in march music.