Only in our second bathroom with one year old three knob jameco tub faucet when giving our daughter a bath the water goes hot and cold even with both hot and cold open all the way it goes warm to cold back to warm,? All other bathrooms and sinks water warmth stays constint,, what's my problem ?

I assume you don't have Pressure and Temperature Balancing faucet...

Check you washers and seats. It seems like one of them is loose.

To do that, you need to close water to the house, remove trim, hot and cold stems and look at the end of the stem where washer sits. Seat is inside the body of the faucet. Take flash light and look inside. Put your finger in and see if it is loose or not.

If that faucet is Pressure and Temperature Balanced type faucet, than you will have to replace the balancing cartridge . Post photo of the faucet so we can ID it.

Back to you. Milo