After my dryer has been running for about 15 minutes, it begins to get really hot and trips the breaker. We are pretty sure it needs a thermostat, but the information shows 3 thermostats. How do you know which one to replace or do you just do all three? And can you do this from the front or do you need to from the back?

It's usually a good idea to replace them all. They're relatively inexpensive, and replacing them all at the same time will prevent your having to go through the work too soon afterwards. this will also give you an opportunity to clean the loose lint out of the dryer cabinet.