As in the 2009 original, Bradley Cooper, Ed Helms, Zach Galifianakis and guest star Mike Tyson will film scenes for the third and final instalment off the Las Vegas Strip and at Caesars Palace, the Las Vegas Sun reports.
Director Todd Phillips is also believed to be shooting much of the movie in Tijuana and Los Angeles.
The film will reportedly centre around the friends attempting to spring Alan (Galifianakis) from a mental institution.
Filming is set to start within a month and the scheduled release date is May 24, 2012.
Phillips confirmed last month that the third movie in the franchise would be the last.
"We're going to surprise a lot of people with the final chapter we have planned," he said. "It will be a fitting conclusion to our three-part opera of mayhem, despair and bad decisions."
Helms had previously described the movie as a "work in progress" and added that the team was "hopeful" that it would come together.