The driver reduces the voltage supplied to the motor winding so the maxiumum current is newer more than 2.8A. Because of back emf in electric motors you need to supply higher voltage when the motor is turning fast. That is beacuse you should use 48V so the driver manages to deliver 2.8A (and thereby sufficient torque) at higher speeds. So maximum theoretical power you get out from a motor is U*I which in your case is 2.8A and 48V = 134W. With 3 axes you have a total maximum consumption of 403W. In practice you need about 50-70% of this because they newer draw maximum at the same time and the real maximum power available per motor will be a bit lower. So the 7.3A power supply is sufficient.
Btw. For best speed/torque curve you should use motor with lower inductance. 2 - 2.5mH motors is nice on 48V power. But the 3.6mH motors will be fine if you don't need maximum speed.