Stepper Motor rotating but vibrating
if your steppters are rated at 2.5A you should be giving them as close to that as possible.

your driver circuit is severely underpowered for 3 motors, with one you are almost maxing out the driver and still only getting 2A. 

if the stepper motors don't have enough voltage they will stutter and not go into the next step. (not enough force to overcome the magnetics)

Either buy a higher power controller (for all 3, 6-8A max output) or  try running one in bridged mode to confirm the power issue.

also .3A power supply will not be able to power any of this.

get a 10w power supply at least for just the motor controller.

List of things that are required upgrades to function:
~12V, 4-6A power supply
~2A per channel motor controller.
use the 300mA power supply for the microcontroller.

Good Luck!