I have two issues. When im driving, the temperture guage will go up and down only when the hear is on. I replaced the theormistat and it still does the same thing. The second is i smell an antifreeze odor smell through radiator but cant find no leaks nowhere.

Best case scenario is your thermostat is stuck open. When this happens the engine will take a long time to warm up and will never get up to full temp. Worst case could be a serious problem such as a blown head gasket. If there is no coolent in the engine the temp gauge will not register either.

could be bad earth onto chassis, also check alternater belt

It is normal for your guage to to fluckurate a bit,

The guage itself is probably bad. Maybe try replacing the guage or instument cluster with a junkyard one.

gm sent out a letter to 2003 owners if you have cluster sticking gm will replace for free

Thermostat will need to be replaced. I recommend draining a quart of anitfreeze by loosening the petcock on bottom of radiator. Then re-tightening. Then follow upper radiator hose, it will lead right to thermostat housing. It should have about 3 screws/bolts on it to remove. The thermostat is underneath.

It is IMPORTANT That you Correct This ,as Engine Temp is Important. I Recomend the Coolant Temperture Sensor Be Replaced to Correct this.

Send cluster out to be checked and or repaired to Model Electronics.

If you unplug temp sending unit and turn ignition on check guage. It should read all the way hot or cold. If so then replace temp sending unit.

Check for an OBII code. have it scanned for free at an autoparts store. it is not the thermostat since you have already replaced it 3 times.Check for the temperature control sensor during scanning..