German magazine Gamestar reveals parts of the Gothic 3 story on their website!

Gothic 3 will start where the predecessor left off - the hero will land his ship at the mainland which is four to five times bigger than the world of Gothic 2. The mainland will consist of rolling hills, stretches of jungle and desert... dunes, oases and oriental cities included. All this could be seen at the E3 in the form of an early tech demo.

The developers haven't decided on the means of transportation yet - horses or camels, everything is a possibility at this point. Water supplies will definitely play a role.
It will chagrin many Gothic veterans to hear that the hero will lose at least parts of the abilities and items he acquired last time - but he won't have to start out as a com! pletely blank slate like he did in Gothic 2.
Factions and parties will play a major role, so much is sure; the following story info contains educated guesses by the Gamestar crew, however:

A side effect of the orc wars, the country's major cities have been taken over by feuding warlords.  The player will probably have to choose between helping one of the warring parties or alternatively save the king and his realm.

Regarding other game elements, Predators will have several forms of attack moves/animations now, like, for example, 'walljumping' off rocks. The AI has been improved to react better to missile attacks, and controls will be mouse-based by default. 

Gothic 3 is supposed to be out by the end of 2005.