md3tomdl is a command line program for doing exactly one thing: converting a md3 file to mdl format.
Version 0.4 fixes an error in the export process relating to MD3 files with a non-zero origin set on the frames, and a crashing bug if a skin file is not specified.
There’s a fairly detailed readme explaining exactly what md3tomdl does and how to use it. It also comes with the source files for generating the gothic arch window model from this site, so you’ll be able to get to grips with it right away. The file also comes with the full c++ source code for the tool.