Because I really can't find any ANYWHERE, and the developers of this game even made a mod client, I believe, FOR the game?

The Last Resort mod:

http://www.mariachimaestro.com/NewSwat3Reunited/download.php?view.33

Kaidane compiled a lot of SWAT 3 maps too:

http://dl.dropbox.com/u/1019015/mappack1.7z
http://dl.dropbox.com/u/1019015/mappack2.7z
http://dl.dropbox.com/u/1019015/mappack3.7z
http://dl.dropbox.com/u/1019015/mappack4.7z

I guess you should just mention that The Last Resort isn't really a mod, but adds support for stuff like proper resolutions et al.

thanks vagabond!

Here's a few mod sites I've come across in my search for mods:

http://www.sierrahelp.com/Misc/SWAT3Addons.html (Contains a few TGOTY extras that GoG's version doesn't seem to have)
http://hotswatmods.com/index.php (Some files require registration in order for you to download them)
http://www.swat3reunited.com/ (A lot of this site's files require registration as well)