On your Mach3 display screen on the left side, where you load a program, there is a line for " start at line number"
enter your line number there. When you press "start from line # you will get a premove and will then move all axis to that position.
Before you hit the OK button , manually move to the X,Y, location listed in the premove. Other wise Mach 3 will move ALL 3 axis to the given destination. Possibly ruining your workpiece. After manually moving to the Premove X,Y location you should be OK to start at your line #.
Or , you could use the MDI screen to move to the premove X,Y destination.
I hope this helps you.