Did you apply the Community Patches? Jowood granted them source code access so they could fix the problems left (which were quite a bunch). They also added tons of additional content and balance fixes. In the meanwhile they also fixed the rather unplayable Gothic 3 add-on...

You can find further infos here (German fan community whose members did the fixes):
http://forum.worldofplayers.de/forum/showthread.php?t=586683

BTW:
I could play it just fine with the initial release patch only...