DB25 -- sorry - that's the connector that you plug the PC into.

Okay, no drama with what you've said so far. I've not used Mach3 for a while so some of this detail may be a bit off (someone will correct me!), but on the Diagnostic page there's indication of the various pins (not necessarily the ones bound to functions like Home - but the input pins 10,11,12,13,14) - observe those with the switches you've added. The silk-screen printing on the board indicates that X is input pin 11, Y=12, Z=13 - just look to see if those change in response to the switch actuation.

If not - pull the sockets from the board and just short the adjacent pins for X (then separately for Y, then Z) - should be evidenced on pins 11/12/13.

If that works then we know the wiring is good, and the rest is just Mach3 settings.