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).
In Rock music an ensemble of electric guitar, electric bass guitar, and drum kit is known as a power trio.
Another use of the word in music is to refer to the middle, contrasting section of a piece in ternary form. This usage gives rise to the "minuet and trio" (or the "scherzo and trio") which often appears as the third movement in a symphony, sonata or similar work.
The trio is also the third section of a march. See march music
A group of three companies or underworld gangs may be called a triad
Three humans sharing the same gestational period are called triplets and are usually all born within a short period of time.