The Maestro’s operating voltage range on VIN is 5-16 V, so three AA cells (which have a nominal voltage of 1.5 V each if alkaline or 1.2 V each if rechargeable) would supply too low of a voltage for it. Four alkaline AAs or five rechargable AAs, giving 6 V, would be a better choice. If you are also powering your servos from the same battery pack, be careful not to use too high of a voltage; most servos are designed to run at 4.8 to 6 V.