Your Aquahot model 450D has only 2 heating zone circulation pumps, a stir pump and an engine preheat pump. Only two thermostats are needed to control the Aquahot coach heat. The stir pump operates when hot water is called for. The engine preheat pump operates when needed for heating the cummins engine block.
Your middle thermostat will only operate the roof middle air conditioner and electric heat pump.
Sammie