G'day all..
OK, I don't check this forum anymore but I've got some time to burn and I remember having this question when I was a newbie myself with Gp3.
1 Install Gp3
2 Download the official version 1.13 patch for Gp3, install it.
3 Download the no cd crack for gp3, install it.
4 Download/Install GpxPatch and Gp3Edit.
if you use Hardware mode (ie have a video card) go to step 8.5 It's difficult finding a software 2002 season set, but I'm pretty sure Ralph Hummerich released one. Have a look over at his site, do a google search on him if you don't know his site.
6 Open the file u downloaded with Gp3Edit, and export it to the game. Depending on the download you got, you may have to find an additional update which exports the driver pics as well. I don't know if RH's set includes them. If they dont, then find '2002 driver sets' or something similar. It will be the same type of file as ur first download. Again, open it up in gp3 edit and export it to the game. All the stuff u exported to the game from the original 2002 RH set will still be there, plus the additional stuff ur exporting.
7 Go to step 9.
8 Download the 2002 update from Ralph Hummerich, and follow all the text files and stuff to use GpxPatch to export it all to your game.
9 Play the game!