noun
A musical ensemble of three instruments, or a piece written for such an ensemble. The term is specific to instrumental music, not vocal trios.
彼らは三部合奏でモーツァルトの曲を演奏した。
They performed a Mozart piece as an instrumental trio.
三重奏 is a more general term for a trio (instrumental or vocal), while 三部合奏 specifically refers to an instrumental trio.
Compound of 三部 (three parts) and 合奏 (ensemble, instrumental concert). The exact historical derivation is uncertain, but the term is conventionally used in music contexts.