A group called the Philadelphia Trio performed a real service at its Phillips Collection concert yesterday by playing the early Trio (1935) by that sage of harmony, Harvard and musical tradition, the late Walter Piston.

Piston wrote the standard text on harmony and educated a whole generation of American composers at Harvard, including Elliott Carter and Leonard Bernstein. As a result, his fame today derives more from his influence on others than from his works. And that is our misfortune.

Take the Trio, for example. It more than held its own on yesterday's program next to works of Schumann and Haydn; it is a work of notable substance, intensity and craftsmanship. Imagine the formal purity of the Ravel chamber works; substitute a more austere harmony, lots of counterpoint and the jagged intervals and syncopations of the Copland generation and you get the idea of the Trio.

It got an alert, lyrical performance from the three women who make up the Philadelphia Trio.

Before the Piston came Haydn's short C-major Trio, Hob. XV:27, one of his scintillating, whimsical works, crafted with great polish and wit. The piano tends to dominate the Trio and throughout the concert, which opened the Phillips' Sunday recital series, Elizabeth Keller played with grace and force on that instrument.

In conclusion, there was the Schumann D-minor Trio. Superficially, it is a piece in the same mold as the famous Schumann Piano Quintet. But in the Trio, his melodic gift does not serve him as unfailingly as in the Quintet. The performance was splendid; some of the pitch problems heard earlier were solved.