Proper installation of a three-way shower
I agree that the setup you wanted has been the way I have seen similar arrangements, and seems intuitive, but the product description for the mixing valve unit says "controls the water flow to your outlet of desire". 

My thought is that a plumber who cares about pleasing his client would have asked prior to roughing it in, but I don't think you have any legal standing. 


  These shiny chrome plated mixer valves do much more than adding a designer look to your bathroom. They are convenient and safe to use with a 3 Way diverter. One knob controls the water flow to your outlet of desire, the other controls temperature and the third control water flow. To protect you and your family from scalding these valves are fitted with leading Vernet thermostatic control. Pre-set at 38 degrees this system prevents any water above this temperature from leaving the shower, for further safety it is enabled with an automatic shutdown if the cold water supply ever fails.