Info on Freeciv can be found at
www.freeciv.org
Best place to find out more is to join the freeciv-dev mailing list. Details are on the site.
There are no Civ3 mods as such for Freeciv. At the current stage of development there is no support for culture, strategic and luxury resources, and a host of other Civ3 engine features. The latest versions have diplomacy similar to Civ3 though.
Graphically Freeciv has no support yet for animated units. The last I checked the developers were doing a major overhaul of the terrain handling code so it should now be almost possible to port Civ3 terrain graphics across. Default Freeciv isometric tiles are 64x32 or 64x48. Civ3 uses 128x64 and 128x96. I'm not sure if this tile size is supported yet. I'm sure if you bug them they might put in support for it, especially if you were willing to convert the graphics.
Realistically Freeciv is an (IMO) improved Civ2. It is quite different to Civ3 and most of the developers aren't fans of Civ3.
Because I own a Mac (and therefore don't have PTW or C3C) I play a modded Civ3 whenever I want to play single player and Freeciv whenever I want to play multi-player. MacOS pretty much has Linux built in.
It's coded for Linux, but there are also versions for Windows, Amiga (which I used to use until my beloved Amiga died after 15 years of blissful happiness

) and a bunch of other platforms.
Lots of fun. Check it out.