A three part valve is provided. The valve has a housing (10) having three axially spaced inlet ports (12, 16, 22) and an outlet port (26). A rotary valve assembly with a modular rotor is provided for controlling fluid communication between the inlet ports (12, 16, 22) and the outlet port (26). The rotary valve assembly includes a shaft (30). Additionally there is provided a first generally cylindrical valve module (32) torsionally affixed with the shaft (30) for controlling fluid communication between a first inlet port (12) and the outlet port (26). The rotary valve assembly also includes a second generally cylindrical valve module (46) rotatably mounted on the shaft (30) controlling fluid communication between a second inlet port (16) and the outlet port (26). The second valve module (46) has a coupling interface (47) with the first valve module (32). A third generally cylindrical valve module (50) is also provided rotatably mounted on the shaft (30). The third cylindrical valve module (50) controls fluid communication between a third inlet port (22) and the outlet port (26). The third valve module (50) has a coupling interface (53) with the second valve module (46).