Double shower diverter?

Hi, I have a three faucet shower/bath that was installed before we moved in. Can't get to the pipes without breaking the titles. The middle diverter faucet is not working, spewing about a half force stream into the tub even when fully open to the shower. I've tried changing the cartridge to an equal pfister one but it seemed to make it worse, i put the old one back and gaskets etc seem fine. I fixed the hot and cold two years back changing out to new cartridges and they work fine, so i assume i have the right brand for the diverter, is there anyway besides the old cartridge to kniw what brand i need? And the odd ball question is; can i just leave it ooen to the shower and use a tub spout diverter? Will that cause any back pressure issues ir what have you? Thanks!