It all depends on the tool changer.
If the ATC requires critical timing, then you'll need to use some form of external controller to act as an interface between Mach3 and the ATC, as Mach3 macros only have a 10hz update rate so critical timing points can easily be missed.
If it doesn't require critical timing, then it's just a case of connecting up the inputs and outputs.
Regardless of how it's connected, you need to write/modify a suitable toolchange macro to handle the required toolchange sequence, via whatever inputs/outputs are being used.