tri·o 
(trēō)
(trēō)n. pl. tri·os
1. Music
a. A composition for three voices or three instruments.
b. A group of three singers or three instrumentalists.
c. The middle, contrasting section of a minuet, scherzo, march, or various other forms.
2. A group of three.
[French, composition for three voices, from Italian : tri-, three (from Latin; see trei- in the Appendix of Indo-European roots) + (du)o, duet; see DUO.]