Is your question about whether a single 3A supply would be enough? If you are not looking for much torque from the motors, then most drivers will have a "max motor current" setting which you could wind down to, say, 1A. I'm assuming single power supply/multiple drivers here, of course. Problem is greater if you are using a switch-mode power supply as these often have a "hard" current limit at their max rating so can effectively switch off just when you need most current at peak load. Old-fashioned linear power supplies are much more forgiving of peak loads. Or wind down max motor current a bit more, or use a bigger or multiple power supplies.