I added two other multiplayers I discovered recently, I'll add more sources when I get less lazy, in the meanwhile some of them can be found by googling, other will need more effort to be found. The problem is that some mods like GTA:T have tons of dead links which can be explored thanks to the Internet Archive, but still few notices about a GTA3 version, for what corcerns GTA:T I'm prone to think the GTA3 multiplayer was dropped soon, it's already hard finding notices about it after 2003, except in the main developer personal site. For what corcerns the newly added YaMM and GTAWO info about them can be found in several topics in the MTA forum. I also found a video showing a start in a possible development of another multiplayer, but sadly I'm not able to find that video again.