I am trying to change my bathtub faucet, it has only one center piece for both hot and cold water, it looks like 3 small copper pipes that hook up to the back of the faucet with a round tin cup that the water stem sets into, how do I get that center piece out to go find one like it at the hardware store?

Take a picture of what you have so we can better see how to help. Also tell us the brand name of the valve and its age. Regards, tom