Slim,
For best noise immunity, you should wire them all separately. For safety, the limits should be directly controlling a relay that cuts power to the axis drives, and NOT depending on Mach3 to be doing that for you, as Mach3 may have lost its mind, and be the cause of the problem. That means having a small board with logic that combines all the switched into a single relay control signal. I'm in the process of building optical limits for my knee mill, and making a PCB to interface the optos to the BOB, and provide the combining logic and relay drivers.
Regards,
Ray L.