Refers to a musical composition for three instruments, or the ensemble itself. Compare with 三重唱 (vocal trio).
彼らはピアノ三重奏を演奏した。
They performed a piano trio.
二重奏 is a duet for two instruments, while 三重奏 is a trio for three, and the similar form makes them easy to mix up when reading quickly.
piano trio
Compound of 三 (three), 重 (layer, fold), and 奏 (play music). The reading さんじゅうそう follows standard on'yomi.