Mach 3 dosen't convert files for G code. . You need software that can use / import stl files and allow for manipulation of the stl file to create G code
in a format which is used by Mach. Not familar with CarveWright, but, if you go to their site you will find info on using it. See if there is a post processor available for Mach3 from CarveWright.
RICH