Tub faucet runs when shower is on.
You have to replace the tub-spout-with-diverter.
It will either screw on, or slide on and be held with a set-screw. If there is a slot in the bottom of the spout near the wall, look for a set-screw with an Allen wrench head to eb loosened so that you can remove the old spout.
If there is no slot, unscrew the spout counter-clockwise to remove it.
Wrap 2-3 flat wraps of teflon tape wrapped clockwise only on the threaded end of the pipe sticking out of the wall before installing the new one.
You should be able to tighten it on by hand or a strap wrench to keep from marring the spout finish.