I have replaced the radiator, water pump and thermostat. The air is blowing hot again, but when I go over 80mph it overheats.
Things that could cause this condition come to mind...
1. The place that did your repair didn't replace the thermostat (even though they said they did). Or the new one was out-of-the-box defective (rare).
How to tell
.... when the car gets hot, is the rubber part of the upper radiator hose where it enters the radiator hot ? (be careful not to burn yourself - do a quick touch). If the hose isn't hot, then the thermostat is not opening to send the coolant to the radiator. Fix is to just replace the thermostat again.
2. The place didn't put the serpentine belt on correctly. The Pentastar 3.6 and 3.2 engine are a little wonky in that you need to take the serpentine belt (which drives the water pump) off in order to get certain hoses off. If they replaced the water pump but didn't route the belt correctly, the water pump could be spinning backward. This would cause the water to not be circulating properly.
How to tell?
There are plenty of diagrams/photos on the internet of the proper routing of the serpentine belt on a 3.6 engine. Look to see if the belt on your car is routed that way.
3. Air in the system. The place didn't fill with a 50/50 mixture of OAT coolant. Sometime they skimp and use any old coolant available (e.g. Prestone). They are not compatible. Also, if there's air in the system and it's not full, this could cause an overheat.
How to tell?
There's an air bleeder screw on the top of the thermostat housing. Unscrew it until coolant comes dribbling out and then close it. Also, there's a bleeder at the top of the radiator. Harder to get out but same process.
There are other things as well (such as they didn't re-connect the cooling fans) but most of the other things would present themselves as overheating idle
, not higher speeds. Do you live somewhere in the US where it's cold or warm right now? (e.g. if you live in Fargo, ND, it shouldn't really overheat even without the fans not working in the winter. Not the same however if you live in Phoenix or Miami of course).