In the 1950s musical polymath Gunther Schuller coined the term "third stream," primarily to describe a synthesis of jazz and classical music. Three and a half decades later the concept is no longer new, but successful examples remain relatively elusive: among music theorists it's this century's version of the Holy Grail. Does that make crusaders of this Canadian-American trio? Maybe; in any case few improvising ensembles steer so close to a balanced fusion of jazz and new-music elements. The trio grew out of the long-standing partnership between pianist Paul Plimley and bassist Lisle Ellis, both of British Columbia, who had worked for years as a particularly adventurous duo. American drummer Gregg Bendian--perhaps best known for his stint with Cecil Taylor--displayed his strong classical background in a solo percussion concert here in 1990; when he joined Plimley and Ellis several months later, his resonant tonal colorings created a vastly expanded stage for the duo's creativity. Like Taylor, Plimley et al create ferocious improvisations from the materiel of the European classical tradition. But this group brings more focus to the music's main components--the clearly stated structures of formalism on the one hand, and unadulterated jazz on the other. Prime stuff. Saturday, 9 PM, HotHouse, 1565 N. Milwaukee; 235-2334.