Hi,
this post should be on the Mach4 General Discussion board. No doubt Tweakie will shift it there in his own time.
Motor3 is the A axis....do you have an A axis? The homing code calls for all axes in turn, it has found and homed the X, Y and Z and then tried to home the A axis, but
you don't have a motor assigned and you get an error.
Go to the screen load script and comment out the line that homes the A axis. This is a very common question by newcomers to mach4.
Craig