In general you want to have a suplly voltage much higher than what you are looking at. Here is some further explanation of that point:
Supply current is a debatable topic. On full and half step (which your L297-8 arrangement is) you will have instaneous demands for 4A per motor per your description, but that doesn't mean you need a 12A supply for 3 motors. As the supply voltage increases the duty cycle of the chopper will decrease. So the average DC current value will decrease. So first decide what voltage you want to run.