noun
vocal trio
Refers to a group of three singers performing together, or the music written for such a group.
彼女たちは三重唱で美しいハーモニーを聴かせた。
They performed a beautiful harmony as a vocal trio.
三重奏 is an instrumental trio, while 三重唱 specifically refers to a vocal trio.
Compound of 三 (three), 重 (layer, pile), and 唱 (sing, chant). The reading さんじゅうしょう follows standard on'yomi.