It is slightly disorienting to go to a concert expecting the Third Symphony of Charles Ives and to hear instead the 40th of Mozart, but nobody in the audience seemed unhappy at the substitution when the Urban Philharmonic opened its three concert series yesterday afternoon at the Peoples Congregational Church.

Conducted by Darrold Hunt, the 36-member orchestra had the right size and balance for the Mozart opus, and Hunt's interpretation gave the music a classic poise without detracting from a special air of spontaneity - almost, as though the symphony were happening for the first time.

The Mozart was not the only substitution on the program. The beautifully crafted, evocative "Night Music" of Howard Swanson was substituted for Ulysses Kay's Overture for Orchestra and played with a fine sense of its texture and atmosphere.

The program concluded with the best of its three performances, Beethoven's First Piano Concerto in a poised, elegant statement with a superb solo by pianist Thomas Flagg. There was a graceful precision in the rapport between soloist and orchestra, a shared sense of the serious playfulness that Beethoven put into this music, and a technical polish that was the highlight of the concert.

The two works omitted yesterday will be played in the final concert June 17, for which the Mozart and Swanson were originally scheduled, and one may conjecture that this music was postponed because more rehearsal time was wanted. This fact pinpoints the Urban Philharmonic's problem. It is right now a good orchestra and potentially an excellent one, but it is underemployed. A busier season and more work together would help to eliminate the small problems of ensemble (in parts of the Mozart, for example) that could be heard yesterday.