The probe function on Mach 3 is fairly straight forward.
The tool is earthed, and when the function is called, the Z axis moves down until the tool contacts a pad, (connected to the "probe" input) and then it stops.
Tool change facility, and the system for writing scripts to control it is subject of a video - see "scripting" . This shows you how to write your own script to make the tool do what you want when you call a tool change.
What you clearly need is to write a script that moves the tool to a position for you to change it, then wait til you change, then move to the pad and drop the tool to get a reading, then change the DRO to a set figure, and move back to where you were.
It sounds a lot, but when you get into it , it is relatively simple. The problem is that everybody 's tool change is different so it is a problem to write one to suit all.