Hi, Skipper
Yes, There called Post's, Mach3 uses Fanuc/Standard G-code as do many machine controller's, It may not support special G/M-code's though.
Fanuc Post may get you going, Post some G-code with it and do some air cuts, But most Cad/Cam Mfgr.'s have a Mach3 Post these day's.
If it won't load in Mach3, In the G-code window it will stop on the White line and at the bottom Status: Line and should show the problem.
Hope this Helps, Chip