These days the chamber trio isn't as popular as the duo or the quartet. But the trio sonata--two strings plus a keyboard continuo--did enjoy a long vogue in the Baroque era. Later Haydn shifted the balance of power in his trio sonatas, expanding the piano part at the expense of the cello. This change encouraged indifference to the form among prima donna cellists, and since then far fewer noteworthy trios have been written than quality quartets. (The same disparity has held true with performing groups.) Three trios--each from a major stylistic period--are included on this program by the Beaux Arts Trio, one of the handful of eloquent exponents of the genre. Beethoven's B-flat Major Trio is a lighthearted romp (originally scored for the clarinet in place of the cello) with a melodious theme-and-variations finale; the piano is the star, yielding center stage to its partners only occasionally. Tchaikovsky's Trio in A Minor is an achingly lyrical, sorrow-laden eulogy for his friend and mentor, pianist Nicholas Rubinstein, written, surprisingly, for an instrumental combination he disliked. Shostakovich's Second Trio (1944) was also intended as a memorial tribute to a friend. Its heavy melancholic air is at times relieved by feigned jollity, but when the music turns macabre toward the end, there's no mistaking the composer's sense of loss. Beaux Arts founding member Menahem Pressler (piano) is teamed with two younger colleagues, violinist Ida Kavafian and cellist Peter Wiley. Monday, 8 PM, theater, Ravinia Festival, Green Bay and Lake Cook Rds., Highland Park; 728-4642.