Modern modding has moved away from directly editing gta3.img . Tools like let you place mods in a separate folder, so you never need to touch gta3.img . However, for texture replacements, the .img file is still required.
A modern alternative with a user-friendly interface and multi-language support. Step-by-Step: How to Use a Gta3.img Zip Gta3.img Zip
After extraction, you should see a file named exactly gta3.img (no .zip extension). Modern modding has moved away from directly editing gta3