trio

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A composition for three voices or three instruments.
• noun A group of three singers or three instrumentalists.
• noun The middle, contrasting section of a minuet, scherzo, march, or various other forms.
• noun A group of three.

from The Century Dictionary.

• noun In music, a composition or movement for three solo parts, either vocal or instrumental, usually without accompaniment.
• noun A company of three vocalists or instrumentalists who perform trios.
• noun A group, combination, or association of three.
• noun In the game of piquet, three aces, kings, queens, or knaves, held in one hand: a counting combination of cards.

from the GNU version of the Collaborative International Dictionary of English.

• noun Three, considered collectively; three in company or acting together; a set of three; three united.
• noun A composition for three parts or three instruments.
• noun 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A group of three people or things.
• noun A group of three musicians.
• noun music A piece of music written for three musicians.
• noun music A passage in the middle of a minuet, frequently in a different key.
• noun Any cocktail made with a spirit, a liqueur, and a creamy ingredient.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the cardinal number that is the sum of one and one and one
• noun three performers or singers who perform together
• noun a musical composition for three performers
• noun a set of three similar things considered as a unit
• noun three people considered as a unit