Shortly, you can only make use of this mod if you are tired of making backups of the
GTA3.IMG file. Instead of modifying GTA3.IMG, you will need to add the mods or modify
the mods in another IMG, and you don't have to worry about your original files.

If this wasn't clear enough:
You don't need to modify your GTA3.IMG to install car, skin, weapon, etc mods. A new IMG file will be used to perform this action, it's name is: overrd.img. This program will use a folder named: ovrd.