PLEASE HELP! I have a 3 Knob shower system, one for hot, cold, and one in the middle to switch water from tub faucet to shower head. The knobs are independently screwed into the wall. The middle knob is not effectively directing all water to showerhead and half of the water still comes out of the bathtub faucet, filling up the tub before it can drain. I've tried unscrewing the knobs as well as the tub faucet with no luck... I don't want to rip them out of the wall and create a bigger problem. From what I understand, this could be a diverter problem... only thing is, I don't know where the diverter is and how to get to it. Do I need to get behind the bathtub wall? maybe I should just call a plumber?

Any advise is much appreciated.