In bathtub with three knobs. Just changed valves for hot and cold water. Now when I turn on cold water, cold water also leaks from hot water faucet.

The facuet wasn't sealed tight enough when it was put in you can use some of that puddy that turns to like a lead substance to plug the leak(avalible at home depot) ,or just get some pliers and tighten the bolt under or behind the knob to turn the facuet. Hope this helps