The director revealed to EW that the final instalment in the comedy series will focus on Zach Galifianakis.
"The Hangover Part III is Alan's story," Phillips said. "He's going through a crisis after the death of his father, [and] the Wolfpack is all he has."
Jeffrey Tambor played Alan's father Sid Garner in the previous films.
The sequel will see the main quartet - Bradley Cooper, Ed Helms, Justin Bartha and Galifianakis - return to Las Vegas, the location for 2009's first Hangover movie.
Phillips added that Part III, which will also see the return of Ken Jeong as Mr Chow, will be "a fitting end to this three-part opera of mayhem and bad decisions".
The Hangover Part III will open in cinemas on May 24, 2013.
Watch Bradley Cooper discuss the Hangover series with Digital Spy below: