Hanogover 3 comes out in May 2013. There have been a few hints dropped about the plot of the new film; the main strand seems to be that Alan (played by Zach Galiifanakis) has been confined in a mental hospital, for reasons not yet made quite clear but to nobody's great surprise. So his friends Doug and Stu have to break him out of there and then go on the run. That's about all we have been told so far - there are also stories that somebody may die in the movie, but there is no clue whether it is a main character or not.
Mainly, director Tod Philips and others involved with the franchise have been keen to liven up the third film. After the success of the first Hangover film, there were a lot of complaints from critics and viewers that the second one was too samey, so they want to make sure that the third film offers something genuinely different.