From the Mach3 manual:
Program G31 X~ Y~ Z~ A~ B~ C~ to perform a straight probe operation. The
rotational axis words are allowed, but it is better to omit them. If rotational axis words are
used, the numbers must be the same as the current position numbers so that the rotational
axes do not move. The linear axis words are optional, except that at least one of them must
be used. The tool in the spindle must be a probe.
In other words, Mach3 can only execute a Touch Probe command on the X, Y and Z axes. It cannot handle a rotary axis.
You may need to set up a custom sensor for the rotary axis and then do the zeroing by hand.
Cheers
Roger