Steve Reich’s music is so rarely heard in Orchestra Hall that the local premiere of a major orchestral compostion of his is welcome news even if it is belated. Three Movements, commissioned by the Saint Louis Symphony Orchestra in 1985, represents the New York minimalist at the top of his form, asserting with clarity and purpose the supremacy of pulse and tonality. The seating of the orchestra is rearranged to accomodate the flow of ever so subtly changing rhythms: marimbas, vibraphones, and pianos are placed directly in front of the conductor, and the strings are divided evenly between both sides of the stage a la Bartok to better convey the slow alterations in harmony. Presiding over the CSO will be guest maestro Michael Tilson Thomas. Tilson Thomas, who no doubt asked for the inclusion of Three Movements on his program, has championed Reich’s cause since the early 70’s, when the experimentalist first attracted notice as a member of New York’s hectic downtown scene. Also on the bill are two interesting early exercises in rhythm and harmony (respectively): J.C. Bach’s Sinfonie for Double Orchestra in E-flat Major and Mahler’s First Symphony. Friday, 8 PM (with “preconcert conversation” at 7 PM), Saturday, 8 PM, and Tuesday, 7:30 PM (with “preconcert conversation” at 6:30 PM), Orchestra Hall, 220 S. Michigan; 435-6666.