Although your request is not stated as a question the inspector is incorrect. The code states: "The trap and fixture drain for an automatic clothes washer standpipe shall be not less than 2 inches (51 mm) in diameter. The fixture drain for the standpipe serving an automatic clothes washer shall connect to a 3-inch (76 mm) or larger diameter fixture branch or stack." The IPC 2015 Code Commentary states: "A fixture drain is the section of pipe that connects the outlet weir of a trap to a stack, fixture branch or any other drain. A fixture drain conveys the discharge of only a single trap and consists of both horizontal and vertical sections of pipe. A fixture drain ends at the point where the discharge from any other fixture is introduced." The installation you describe complies with the code section as the fixture drain extends to the 3" horizontal piping. If for instance you installed a fixture drain for a laundry sink in the same room on a common vented waste stack with the washer drain, the stack would be required to be 3" at the point of connection of the second fixture drain. This appears to be a common misinterpretation by inspectors as the definition of a fixture drain must be considered.
