If the rated current of the motor is 500mA then thats what you should supply to it, if you give it more you may get excess motor heating, poor linearity and premature failure of the motor. If you were using three of these motors then your power supply should be rated to deliver 2/3 of the total current ie 1A. I have three 6A 2.0V rated steppers on my machine. My power supply delivers 60V dc at 12A to three gecko drives, these are then current limited to 6A with a resistor.
Hope this helps a bit!