GTAMP.com

Updated 2008-04-22

Grand Theft Auto 3 doesn’t officially have any multiplayer modes but some fans really wanted it to, so they created their own applications that added a primitive (buggy) multiplayer.  Most of these no longer work or never really worked well in the first place.   There are much better multiplayer mods for San Andreas.

Unofficial GTA3 multiplayer applications

GTA3 GGM - v0.4 multiplayer mod for GTA3 (no updates since 2003)

There are some others but most have been removed from their original site, were extremely buggy or no longer work.