Please provide a link or links to the actual motors.
Trying another power supply of the exact same capacity wont really prove anything.
Best bet is to remove all but one driver board and verify one motor works for a period of time without issues.
Swap the motor and driver and try again until you have tried all 3 motors and boards in isolation.
That rules out a driver boards and all the motors and the GRBL configuration.
Also ensure you have the correct stepping etc set for each motor in the GRBL configuration
If that goes well add another driver board and motor and repeat the experiment with two motors.
If there are any issues I would still suspect a power supply issue.
With no issues move on to all three motors.
Its a process of elimination.
GRBL does not work "out of the box" and there are thing to set before you use it other than the driver board voltages and even those are I find more of a guide and may need to be tweaked a little to provide additional torque for some applications.