Take a look at this great mod for GTA 3 its well cool changes all the cars and also adds more ramps and stunts. Check it out @ Justcdr.net by click on the screenshot. Files are now on for download.

Update From Digitalwanderer You can get a heck of a lot of cool GTA3 mods & tools at www.gta3hq.com with tons of cool custom cars/tracks, and they come with an easy to use auto installer. :)