Alan (Zach Galifianakis) is driving down a freeway in his convertible, pulling behind a trailer that happens to contain a giraffe. His licence plate spells DADSBO1, he's drinking a beer and he's listening to Hanson's MmmBop. Then a low-bridge decapitates the giraffe, causing a massive pile-up behind him. Alan looks back at the scene he's caused, and sighs disappointingly; there's a chance this minor malfunction may ruin his day. The Hangover Part Three's opening is a pretty good microcosm for the movie as a whole; excessive, ridiculous, dark... if not all that hilarious.
The fallout of this incident sees Phil (Bradley Cooper), Stu (Ed Helms), Doug (Justin Bartha) and Alan's family stage an intervention, and the Wolfpack reunite to drive him to a mental institute. However, along the way they're run off the road by Marshall (John Goodman) and his gang, who kidnap Doug, warning the lads that they will kill him unless they deliver Chow (Ken Jeong), who has stolen millions of dollars' worth of Marshall's gold. And so the manhunt for Chow begins, taking the pack from Tijuana back to where it all began; Las Vegas. Once again, carnage ensues and a trail of craziness and destruction is left in their wake.
In terms of comedy, the first movie was the equivalent of a crazy, fun night out (which, in hindsight, may not have been as great as you remember it), and the second was a nauseating repeater of the first - an actual hangover of a movie. The Hangover Part Three is when The Fear has set in; gone is the blackout that needs to be pieced together, gone are the weddings and gone is most of everything that we thought we knew about the series. Part Three drops the mystery element and delves more into thriller territory, with the comedic aspects still present, but with a much darker slant to them. Alan's "zaniness" is now being dealt with as an actual mental illness while Stu's "experimentation" in Bangkok leads to a joke about getting tested for AIDS; you'll still laugh, but you won't feel good about it.
The Wolfpack now know their roles inside out; Cooper is still handsome and cool, Helms is still a bit shriek-y and confounded, Galifianakis is still out-there and OTT. However, Jeong gets some vastly increased screen-time, and his Chow can go from hilarious to hugely irritating in a heartbeat, so your opinion of him may dictate your opinion of the entire movie.
If this is to be the last Hangover, and a mid-credits sequence reveals it may well not be, they've certainly gone out with a bang, when compared to the second movie. But you can't help but feel they should've just left it as a one-night kinda deal, instead of trying to make a session out of it.