I think the key point is that the coolant doesn't seem to be circulating. If you don't have hot water in the top of the radiator when the engine is hot, you don't have circulation. Without circulation, you will overheat. So either (1) there is an obstruction, (2) the water pump isn't working, (3) the thermostat isn't working, or (4) there is enough air in the system (doesn't take much) that the water pump can't overcome the head pressure. In terms of simple, I'd start with a thermostat that's jammed in the full open position to rule out that possibility.
Both Curt and Dimitri have avoided that simple test (blocked open thermo) like the plague, insisting that 3 thermostats can't all be bad, but experience shows 3 bad thermostats in a row is indeed a possibility. 10-12 years ago 4 bad out of 5 was not unusual. Common, in fact. Then they got some better, but in the past few years, all production is in 3rd world countries, so buying MB or Bosch is no longer a sign of quality. It makes no sense to me to avoid a simple test that will yield a definitive answer.
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