Frank- Ah ! The old three handle valve. Now were getting somewhere … Your seat idea is a good one. Shine a flashlight in there or use the tip of your little finger if you can reach the seat. Any corrosion, rough edges or pitting, replace the seat Some people try to dress the seat, use only as a last resort. To otherwise identify the faucet, take the handle and stem to any good plumbing hardware store Take the seat too if it is bad. A few other suggestions : Take off the shower head. If you get good shower flow with it off…….. If you dont, and the seat is good, the obstruction could be in the shower riser, the pipe between the diverter and shower arm. Let me know how you make out.t.