Can a spout with a pop-up knob be used to replace a normal spout that didn't have a pop-up from the beginning ?

Mr. Clean a diverter spout can be used, but it could cause problems if your "pop-up" ever gets pulled up while your valve diverter is being used. It's definitely better to replace the spout with another non-diverter spout, but as long as care is used, you could probably make it work with no ill effects.

I don't see any problems in installing a diverter spout along with your three handled tub faucet. This gives you two ways to divert from spout to shower. There should be no conflict whatsoever between the two since all the diverter spout does is to close off the spout and that's exactly what you want it to do. Good luck, tom