The piano trio was invented toward the end of the Baroque era as a vehicle for ostentatious court pianists. Haydn, experimenting with the genre several decades later, elevated the violin from supporting role to almost the piano's equal, and Mozart too wrote his trios as rich musical dialogues between the two haughty instruments. The cello had to wait until the mid-19th century and the piano trios of Schumann and Brahns for its emancipation, and by 1892, when Saint-Saens composed his second trio--a brilliant affectation of suave nonchalance enlivened by touches of the salon--this tried-and-true menage a trois was on its way out, displaced by more modern arrangements. "The Evolution of the Piano Trio" could serve as the title for the Chicago Ensemble's first recital of the season: the program consists of one of Haydn's 32 trios (in E-flat Major), followed by Mozart's Piano Trio no. 5 in C and then by the aforementioned Saint-Saens. Now sliding gracefully into its 15th year, the Chicago Ensemble--one of the city's better chamber groups--can attribute its survival to founder Gerald Rizzer's tenacity, its wonderfully eclectic programming, and the respect and rapport among its players. Saturday, 8 PM, International House, 1414 E. 59th; Tuesday, 7 PM, Three Arts Club, 1300 N. Dearborn; 509-9031.