The best progressive triangular formation typically consists of keyboards/bass/drums, alongside Emerson, Lake & Palmer. They are merely a more underground version, but equally valuable, at least regarding their first album.
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Quatermass

Quatermass were an English progressive rock trio featuring John Gustafson (bass, vocals), Peter Robinson (keyboards), and Mick Underwood (drums). They released one self-titled album in 1970, noted for its keyboard-driven sound; Rainbow later covered Black Sheep of the Family.
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