Yes, only consider Gothic 3 if you get the Community Patch 1.73. It makes a world of difference, you'll ruin a nice game for yourself if you even try it without the patch.
The game itself, well, some people who liked Gothic 1 and 2 didn't appreciate G3 all that much. I for one, did. Almost as much as G2 which still stands as The best RPG i've spent my time with.
The list of similarities with Risen is long; level-up system, alchemy, smithing, dialogs, NPC behaviour even down to them making gestures while conversating… etc etc.
Combat on the other hand is different. Risen seems to have a more "intelligent" combat system, more enemy tactics etc. Although the Community Patch does make enemies make use of that in G3 to a certain degree, as well.
It's well worth finding and playing, that's my opinion.