In this epic final installment, 8 years after the 2nd film, Phil, Stu and Alan are heading to Tijuana for a wedding, but they have mutual problems, Phil is about to get divorced by Stephanie on his presumed actions in the last 2 films, while Stu is about to get humiliated by his ex, Melissa, under the drugged cheating ways and Alan has been off his medication and out of control on his father's death, and one big problem: the wolf pack's friend, Ernesto is back for revenge on humiliating him at Doug and Tracy's wedding and Phil's son, Eli is missing after a crazy night.
- Phil Wenneck (Bradley Cooper) - about to get divorced on why he hasn't told Stephanie about his crazy night
- Stu Price (Ed Helms) - about to be humiliated by Melissa on his past drugged affairs with women as revenge, in the next morning, he realized he lost his left leg to a grizzly bear
- Alan Garner (Zach Galifianakis) - has been off his medication and out of control after Sid Garner's death, revealed to have got the drugs from a store in Vegas
- Doug Billings (Justin Bartha) - now a father, he is still loved by Tracy due to his promise and despite his craziness in the 1st film after Marshall exposed everything Alan has done in the past
- Tracy Garner-Billings (Sasha Barrese) - Doug's wife and Alan's sister
- Eli Wenneck (Joel Courtney) - Phil's son, who decides to live with neither of his parents upon their near divorce, he later joins his father, Stu and Alan on a crazy night, when they walked droopily back to the motel drunk, Phil, Stu and Alan were knocked out by Marshall's thugs, and Eli was left kidnapped as Marshall's collateral and knocked out by "Black Doug" for wrecking Marshall's house and for drinking too much diet coke peeing in Marshall's pool
- Phobe Wenneck (Alexys Nicole Sanchez) - Phil and Stephanie's daughter and Eli's baby sister, who may live with Stephanie after their presumed divorce
- Stephanie Wenneck (Gillian Vigman) - Phil's wife, who questions Phil on his crazy actions in the 1st and 2nd film, and is about to divorce him
- Lauren Price (Jamie Chung) - Stu's wife, who understands Stu's drugged past with women in the 1st and 2nd films
- Maggie and Frannie Billings (Taylor and Claire Geare) - Doug and Tracy's daughters, who didn't know of their father's drugged past in the 1st film, but they forgive him after Marshall exposed Alan's druggings, and their father was sober for a long time
- Sid Garner (Jeffrey Tambor) - Alan and Tracy's father, who is fed up with Alan's crazy behavior, after arguing with him, Sid dies of a heart attack, while Alan was listening to Billy Joel's, "My Life"
- Linda Garner (Sondra Currie) - Alan and Tracy's mother
- Melissa (Rachel Harris) - Stu's ex-girlfriend, who threatens to humiliate Stu by exposing his drugged affairs with Jade the Stripper and Kimmy from the 2nd film, she was unaware that Stu was drugged cheating on her
- Marshall (John Goodman) - the main antagonist and "Black Doug"'s boss who was mentioned in the 1st film, he was aware that Phil, Stu and Alan were connected to Chow, who stole $21 million in gold from him (Marshall stole $42 million in gold from a sheikh and split the cache into two hauls, one of which was then hijacked by Chow), and had threatened Eli's life and that Alan was the only to communicate with Chow, Marshall exposed Alan and made a video of him drugging Phil, Stu, Doug (in Vegas), Teddy (in Bangkok) and Eli (in Tijuana), causing their drugged ways, as a message to the government and the president revealed this information
- "Black Doug" (Mike Epps) - revealed to be Marshall's head of security, was killed by Marshall for his incompetence
- Ernesto the Besto (Hollywood Yates) - the secondary antagonist and was humiliated by Phil, Stu and Alan at Doug and Tracy's wedding in '09, by getting pantsed, when he attempted to propose to his girlfriend, Julie, who ends up dating Stu after he ended his relationship with Melissa, he became mad at them and swore if he saw them again, he would break their arms and yank off their ears, his quote is "Wolfpack, I'm gonna destroy you!", in the wedding in Tijuana in '17, he was messaged by Eli to be invited to the wedding, which shocks Phil, Stu and Alan, as they told Eli what had happened at Doug and Tracy's wedding, Ernesto then banged down the front door attempting to kill them, but was reasoned by Eli that they wanted to make it up to him with the song, "Eye of the Tiger", by Journey, which made Ernesto happy with the song and accepted their peace offering, but warns them if he doesn't like the song, he will break their arms and ears off, then when Alan had drugged himself with Phil, Stu and Eli, they humiliated Ernesto again with the same thing, as he angrily chases them into Phil's minivan, and was arrested by Tijuana cops, when Alan attempted to reason with the cops, he accidentally ripped off Ernesto's pants, then in the morning he came back for more revenge against them for humiliating him again
- Julie Montez (Liv Tyler) - Ernesto and Stu's former girlfriend who went on to marry Issac, ever since she never married Ernesto during Doug and Tracy's wedding
- Issac Montez (Whilmer Vladerama) - Julie's husband ever since she broke up with Stu
- Leslie Chow (Ken Jeong) - had been in Bangkok prison for 7 years and was questioned by Marshall about the $21 million, but he refused to talk, he had been hiding
- U.S. President (Ronny Cox) - seen on TV revealing information about Alan's drug plans, courtesy of Marshall
- Chief Officer Morgon Shortshirt (Colm Feore) - the one who interrogated Alan on where he get the drugs
"Since the events in Bangkok, Leslie Chow was arrested by the Bangkok Police Department for the drug dealings, he was ordered a 10-year sentence to recover the drugs he made for the people he drugged"
6 years after the events in Bangkok, Leslie Chow (Ken Jeong) escapes from a maximum security prison, using a riot as cover.
In America, Phil Wenneck (Bradley Cooper) is having dinner with his family, his wife, Stephanie (Gillian Vigman) wants a divorce because of some events while Phil's wild time with the Wolf Pack at bachelor parties. Even Stu Price (Ed Helms) is having a worst time, when he realizes his ex-fiance, Melissa (Rachel Harris), comes back by threatening to humiliate Stu, by exposing some pictures of him having affairs with Jade and Kimmy, so Stu has no choice but to reveal to Lauren (Jamie Chung), his wife, that he had affairs during bachelor parties with the Wolfpack, and that Alan had drugged him, which Lauren understands, then calls Phil and Doug (Justin Bartha) about this. Phil hopes Alan (Zach Galifianakis) is having a great time.