Multi Theft Auto:
GTA3: MultiTheftAuto 0.3b
Turn your Grand Theft Auto 3 into a multiplayer game with MultiTheftAuto. MTA is the first multiplayer mod released for GTA3, it supports up to 36 players and new game modes. Download link is approximately 1/4 down the page. Installation directions and instructions are included in download.
Other Mods included at this site are vehicle mods(flyable helicopter included), weapon mods, sound mods and various other mods. Dozens and dozens of mods, just make sure to translate site to English since it's a German site.
Grand Theft Auto Real Life Mods:
Real Life Mods includes mostly graphic mods such as changing the apperance of gas stations, cars, resturants, and much more. There's also a mod pack that can be downloaded that offers weapon mods, audio editor and more.
IGN Grand Theft Auto 3 Mods & Downloads:
This page offers MultiTheftAuto, RealGTA3 Mod, SpiderMan Skin, and a number of other Grand Theft Auto 3 downloads.