triad; common chord
In music theory, a chord consisting of three notes: root, third, and fifth. The term 'common chord' is an older English equivalent.
Triads are the foundation of harmony.
This piece is composed entirely of triads.
和音 is a general term for any chord, while 三度の和音 specifically refers to a triad built on intervals of a third.
Compound of 三度 (さんど, 'third (interval)') + の (possessive particle) + 和音 (わおん, 'chord'), literally 'chord of thirds'.