According to The Hollywood Reporter, negotiations for the second sequel have extended for months due largely to the the three primary actors, who are presenting a united front in the discussions.  Each is looking to earn about $15 million, as well as back end gross participation, which they also received on The Hangover Part II.  And apparently the studio will probably make with the cash.

To put that in context, each star earned less than a million dollars for the first film, and their negotiations on the second allowed each to get $5 million plus gross participation that is estimated to have now climbed into the teens.  For even more contextualization, The Hangover's production budget was around $35 million, meaning that the stars' salaries alone would amount to a budgetary figure ten million greater than that unexpectedly huge first film.

The studio is hoping to start production on the second sequel this summer in order to have the film ready for Memorial Day 2013, repeating the release patten that has worked well so far.  With the Harry Potter series finally over, Christopher Nolan's The Dark Knight trilogy closing out this summer, and Green Lantern having failed to light the world on fire, this franchise is of particular importance to Warner Bros.  "It’ll happen,” one anonymous source tells THR. “It’s a proven commodity, which is rare these days.”

Despite being an unqualified commercial success, The Hangover Part II earned a lot of criticism from audiences for following too closely to the story formula that worked on The Hangover, simply transplanting the action from Las Vegas to Thailand.  It's said that the third film, which will take place in Los Angeles, will deviate from the "Wait, what happened last night?" story of its predecessors.

Screenwriter Craig Mazin, who contributed to the script for The Hangover Part II, has been at work on the new screenplay since around the time of the second film's release.  Presumably, he's been working with Todd Phillips, who directed both previous installments.