The NC file is the file which contains the gcode. I'm fairly sure it can have a few different extensions but contain the same information. So it could be myshape.nc myshape.tap myshape.txt and I'm pretty sure Mach3 will open it and read it as they are all basically text files.

But before changing the extension what for a second opinion (!) as I haven't dabbled at that level for many years.

Possibly you are asking a different question ?