1. Download the pack from the link above
2. Unpack .7z file with 7zip / winrar or any other archiver.
3. Go to your emulator folder -> textures -> SCUS-97102 (this one will be different if you use other version of the game) -> replacements.
Screenshot with the path.
3.1 If you don't see SCUS-97102 folder start a race, dump textures by going to
advanced graphics settings then turn it off. Needed subfolders should appear in texture folder.
4. Drag and replace replacements folder.
5. Go to advanced settings and check "Load textures" option.
If you prefer video form (yes I know it's GT4 tutorial but the method works for GT3 as well) -
Link.