The shower has three valve stems, the problem is the shower diverter stem doesn't seem to divert all the water from the tub, so I have both the shower and tub faucet with lots of water coming out at the same time. I replaced the diverter valve stem with the same one I pulled out and a new seat. Why did it not fix the problem.

You may have debris inside the diverter port, close both Remove the diverter stem and look inside if the seat is properly installed, check the seal for damage, now turn on the water hot and cold water to flush out the faucet if there is any dirt and install the diverter stem check and see if it is fixed, if not it seems that you diverter stem is not the correct typ, good luck.

John