I got a lot, cuz I love GTA3, i have it also for PC, and these are the best mod sites

[url:25db1]http://gta3hq.gamecentrix.co.uk/[/url:25db1]

this one is in German but its still pretty good
[url:25db1]http://gta3.gamigo.de/[/url:25db1]

And this has some pretty good texture mods
[url:25db1]http://www.gta3.gamehorizons.net/[/url:25db1]