Replacing old shower faucet

[primuspaul]

I have one of those old in-wall shower faucets where three valves (L: hot, C: faucet/showerhead R: cold) control either the faucet below or the showerhead above. I want to replace them and was wondering if there was some sort of catalog of parts I could look at to try and identify them. I wish I took a picture while the part was out. Does anyone know if there are "compatible" parts that can be installed?

Also, is it possible to somehow incorrectly reinsert the valve cartridge and have it leaking along the inside of the walls?