Ok, so first of all there is a
GP3 FAQ where you can find a lot of useful tips to play and mod the game. Read it, especially parts 2, 3 and 4, it's really interesting.
Now, let's assume that you have installed GP3 out of the box. You need to patch it to use the mods made by the community :
official1.13 patch and decrypted-exe (
links here) GPxPatch (
link here)
Here is a
good step by step to install everything. It is for Seven but runs quite the same way for XP (btw GP3 seems to run in a better way on XP and you don't need to set compatibility modes for it).
So now, how to mod the game ?
First have a look in your GP3 directory (C:\Program Files\Microprose\Grand Prix 3). There are several files and folders. One of them is circuits in which you have several .dat files. Those files are the original tracks of the game. Two other folders are Gp3jams and Gp3jamsH that contain the graphics of the tracks and the game. If you go into one of them you will see several folders with a strange name (Bar3jam,...) these folders contain the track graphics.
Now have a look at a track you want to add : you have a zip/rar file that you must uncompress. In it you *should* have a dat file, a jams folder (or two) and a GP3Form file. The latter isn't always there unfortunately but gets useful notes on how to install the track. The main thing is to know where to put the files. The dat one can be put in the circuit folder, but as I've said previously it's better to put all the tracks you add in a separate folder somewhere on your hard drive. For the jams files they MUST be into the Gp3jams or/and Gp3jamsH folders. The best way is to copy the whole jams folder of your rar/zip file into that directories. Be aware that your jams folder doesn't contain another folder (if it's the case, it's this one that you must copy into the directory).
Now how to use your new track in game : in your GP3 directory (C:\Program Files\Microprose\Grand Prix 3) double click on the GPxPatch.exe you have previously installed, that will launch GPxPatch. Go to the GPxTrack tab and go to the track manager and Edit. There you have all the tracks in their running order. To add your track you have to replace an existing one, so click on the button to change the track and go to the folder where your track is. Save it and launch the game by clicking the Start button in GPxPatch.
That's it.