That depends on a few factors. In most cases, three 3.0 amp motors and drivers will not exceed 8.8 amps of available current. All of the motors would need to be at full torque to reach the 9 amps (3x3). This depends on the mechanical method you are using for each motor, and how you are using the machine. In most cases, only two motors are moving at a time (i.e. 3D carving where the X and Z are moving simultaneously and the y only moves to go to the next row). In traditional cutting profiles, only the x and y are moving simultaneously and the z-axis moves at the location it needs to start cutting the next layer.
With that said, it is ok to add an extra power supply if you feel you need more available current.
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 Would the single 8.8A power supply be undersized for 3 6.0A Drivers? would you recommend getting another PS?