I had my radiator replaces last week and thermostat and my car is still overheating...what could be the problem?

Next is water pump. Take the cap off cold and start and allow to warm up. Watch to see if there is circulation. If not, either pump or a blockage somewhere else, like thermostat installed backward?

could one of these still be the problem even though its overflowing in the overflow and running out of it?

Now that could be a bad cap and yes to your question

really...what is a purpose of the radiator cap with the cooling besides just staying on the radiator

It is spring loaded, to release excess pressure in to overflow tank if weak releases too soon

ohhh...ok...so if that is the problem then the engine should stay cool and of coarse no longer overflow...am i correct?

Yes. If it's weak it's letting coolant out...= low coolant = overheating

ok i will try that and see what happens when i buy a cap for it...

It's an inexpensive try...hope that's all it is, good luck