Looks more like lost steps to me. Backlash would have the circles concentric but not circular.

You could try it at a lower feed rate to see if it's lack of power in the steppers.

A more scientific test is to put a dial gauge against the axis, creep up to a sensible setting and then rapid traverse away from it then come back to the same position at a low feed rate. The dial gauge should have the same reading.

If it's happening on all axes with no pattern it might be noise or if it's happening when doing moves on all axes it might be the power supply struggling.

Have you tuned the top speed and acceleration settings of Mach3? What are you using for steppers / drivers / power supply and what feed rates were you using?