Am I right in thinking by A3 you mean the machine travel is about the size of an A3 sheet of paper? If so the 3nm motors will be plenty big enough. Are you using timing belts and pulleys?

For such a small machine it may not make much difference, but in general you want to use higher voltage stepper motor drivers. The ones on that site appear to be only 30 volts which will significantly limit the torque of the motor at high speeds since the torque at high speed (and thus the speed overall) is roughly proportional to the voltage. This is because the inductance of the motor limits the current into the motor and a higher voltage helps overcome that limit. Those motors work very well on 70 volts and pretty well on 50V. To do that you'll want a driver like this one:

http://www.zappautomation.co.uk/prod...8e24ca2d87b9b3

I'm sure we can help if you're not sure about anything with the wiring.