Mach 3 is programmed to accept pulses into the computer to measure the spindle speed. There are two methods. One is a simple one pulse per revolution of the spindle. The other is several pulses per revolution (these can be specified).
What you need to know, therefore, is the system your existing lathe/mill uses. This needs converting to a 5 volt signal, and then can be fed into your computer.
So the question is - what signal does your system give out.
It may be, however, that the problems trying to integrate an existing system into Mach3 is too expensive. You can buy optical encoders for a few pence/cents and achieve a satisfactory input into Mahc 3 . There are several posts on this if you do a search.
I use an optical encoder to measure the spindle speed and a Digispeed PWM converter to run my lathe/mill with excellent results.
I have attached a diagram for the optical encoder - which can be wired directly to your brealout board. I was looking for a picture - but I can't find one
The tracking of your spindle speed is quite simple - I think the "control" of your spindle speed will be the most important.