No need to get in panic and reinstall the whole game.
GP3 uses two types of files for the wheels - sw jams with bmp inserted ( tyres seen from outside ) and bmp for cockpit view.
Usually installing by GP3Edit but can be installed manually.
If you use GPxPatch you can find /change the tyres in Edit carset window or in gpxpset.ini file.
Simply you should have bitmaps for the tyres in Gp3Jam
sh\Techlam with the
same filename as tyre jams you use place in Gp3Jam
s\Main directories.
If you don't have the same named bmp , the game takes the bitmaps from carskins.
Though, if you ran in software mode, I cannot exactly say whether it works, 'cause never try this way.
At least, in GP2-2000 hardware mode it works.