When following the Docs, this would actually be the Z axis for an MPCNC. There is another thread where someone had to disconnect the power for Z to stop homing because it was going up like a rocket.
I played around with it some more and I did not have the endstops connected at all when doing this. When pressing the button for Home all I got four error messages (X1/X2/Y1/Y2) that no endstops were connected, but Z started homing (and never stopped, like for the person in the other thread). The same thing happens when triggering them one by one. No endstop connected on all of them but Z that starts going and never stops. It is also the third driver, so Z in my case. Same thing seems to happen to @Jonathjon (see one post above).
Conclusion: Something is configured incorrectly for the third slot, since it is homing without an endstop connected (and it does not recognize a triggered endstop (maybe).) So some kind of check is missing here (but I don’t know what, that’s your job to figure out… 
).
/Edit: I just figured out it is not connected directly to the problem in this thread, it is a problem nevertheless for all Primos.