HELP!!!! Old Price Pfister 3 handle shower diverter valve

[SashasPop]

I'm at my wits end. Older 1970's mobile home, and I cant get behind the shower without draining and removing the water heater and cutting into the back wall. This is an old style Price Pfister diverter valve thats no longer diverting. Can anyone help with parts or ideas? Havent showered in a week.