Mach3 is not a closed loop control, so it can't do anything with the encoders except display their position.
There is an aftermarket board and Mach3 plugin that can read encoder info and stop your machine if it loses position.
www.SuperPID.com - Super-PID Closed-loop Router Speed ControllerWhat relay board do you recommend for switching the router on & off
And I'd get the 381oz motors from Keling. shouldn't be more than a few $$ more and will perform much better.