Steve,
The generator can handle the total load of the coach (may include Aqua Hot Heating element also etc) but when you add the starting current of the third air unit, with the other two running, the voltage must drop enough momentarily for the Xantrex to take itself offline, or simply will not operate due to low AC voltage or momentarily not producing 60 cycles AC. Once the starting current has been satisfied and the total current drops back, the voltage rises again and the Xantrex then comes back online. I cannot tell you for sure if this is exactly what is happening but it makes sense based on your issue. I do not think there is a real issue here if the Xantrex comes back online. You may be able to help the situation by turning off the electrical element in the water heating system at the breaker panel if you have one while you run the air units on the genset.
Later Ed