What to do for my 2002 buick rendezvous

You need lower intake gaskets. I'm sure you're losing coolant that you can't see leaking outside the car that's because it's leaking inside the engine through the gaskets. When you buy the gaskets by the good metal ones, don't go cheap. It'll last you in the long run.

Also, seeing as you have a 3.4 motor, it's best to get the heads resurfaced and checked for cracks or bent valves at a machine shop. If you don't, it's a good chance you'll run into the same problem because the heads are aluminum and known to warp.