Belini's Magny-Cours 2003 track worked allright for me in GP3.
Just make certain that you follow the installation instructions and install all of the necessary files including the Belini v2 GenJams. You will also have to insert GP3 magic data into the track.
abcdefg made a Magny-Cours 2003 track that can be downloaded from here:
[
www.freewebs.com]
It has the new 2003 final section, but the original game graphics and the old pit exit and similar pit entrance.