Classical music 
Instrumental ensemble 
In classical music, and more specifically, chamber music, 'trio' may denote a piece for three instruments or voices. Common forms of such trios include the "piano trio" — piano, violin and cello — and the "string trio" — violin, viola and cello.
'Trio' also occurs in the name for the musical form trio sonata, which was popular in the 17th and early 18th centuries. A trio sonata is written for two solo melodic instruments and basso continuo, making three parts in all, hence the name trio sonata. However, because the basso continuo is usually made up of at least two instruments (typically a cello or bass viol and a keyboard instrument such as the harpsichord), performances of trio sonatas typically involve at least four musicians.
Musical form 
'Trio' also refers to the central part of a piece in ternary form (ABA). This meaning is found in the terms "minuet and trio" — or "scherzo and trio" — which often form the third movement of a symphony, a sonata or similar works.
Rock and rap music trios 
In rock music, bands most typically have four to five members, but there are also many three-person bands. The archetype of the trio in rock music is the power trio, having a guitar, bass, and drums, with one or more of those performers also singing. Notable examples of trios following this pattern include Rush, The Police, Motörhead and Nirvana However there are other variations, for example Emerson, Lake & Palmer, where the guitar is replaced by a keyboard. Operating as a trio allows the musicians involved to showcase their skills, as they are collectively carrying the burden of not having the typical fourth member. Jimi Hendrix, although often remembered as a soloist, often performed as part of a trio.
A trio may also rely on a combination of instrumentation and vocal harmonization. Crosby, Stills, & Nash began as a trio noted for their vocal congruence. Some trios rely entirely on the singing skills of the performers working in front of pre-recorded instrumental tracks. These include groups like Destiny's Child and TLC. In rap music, a trio may be composed of three performers who alternate vocal parts, as with The Beastie Boys, The Lonely Island, and Foreign Beggars, or may be two vocalists along with a DJ who provides the instrumentation through sampling and turntablism, as with Salt-N-Pepa, composed of two vocalists and DJ Spinderella.
Common forms 
Common forms of trio include:
- Trio sonata
- Piano trio (piano, violin, cello)
- String trio (violin, viola, cello)
- Flute, viola and harp (flute, viola, harp)
- Jazz trio (piano, bass, drum kit)
- Clarinet-violin-piano trio (clarinet, violin, piano)
- Clarinet-viola-piano trio (clarinet, viola, piano)
- Clarinet-cello-piano trio (clarinet, violoncello, piano)
- Power trio (electric guitar, bass guitar, drum kit)
- Harmonica trio (chromatic harmonica, bass harmonica, chord harmonica)
- Horn trio (violin, horn, and piano)