SPOILER:
Ending, Innos: You give the artifacts to Innos, kill the Chosen of Beliar (Zuben) and the kingdom is saved. According to the ending, King Rhobar II will rule for quite some time, and after that you (the player) will succeed him as the new King of Myrtana.
Ending, Beliar: You give the artifacts to Beliar, and kill the Chosen of Innos (King Rhobar II). After that you also have to kill Xardas, the king of the orcs, and every other orc leader (Beliar is evil and considers all these folks traitors and you the true chosen one). After this, you rule the world.
Ending, Xardas: You destroy all the artifacts, kill the chosen of both Innos and Beliar (Zuben and Rhobar), and gather their scepters. With those scepters you and Xardas open a portal to an unknown world, and go through it. By doing so, you close all contact between the gods (Innos, Adanos and Beliar) and the mortal realms, so the humans and orcs can live without interference from any gods. According to Xardas, this is the only way to permanently stop the war between Beliar and Innos, where countless lives are constantly lost. This is most likely the "true" ending of Gothic 3, and the one the add-on will be built upon (my guess is, the add-on will be in this "unknown world").