The Expendables—led by Barney Ross and formed by Lee Christmas, Gunner Jensen, and Toll Road—extract former member Doctor Death, a knives specialist and team medic, from a military prison during his transfer on a train. They recruit Doc to assist them in intercepting a shipment of bombs meant to be delivered to a warlord in Somalia. Arriving there, they reunite with Hale Caesar, who directs them to the drop point, where Ross is surprised to find out that the arms trader providing the bombs is Conrad Stonebanks, a former co-founder of the Expendables who went rogue and was presumed dead. In the ensuing firefight, The Expendables manage to kill all but Stonebanks, who shoots Caesar twice. As the team attempts to aid him, they are forced to retreat due to Stonebanks’ advanced weaponry, and Caesar is severely injured in the process.
Back at the United States, CIA operative Max Drummer, the Expendables’ new missions manager, gives Ross a mission to capture Stonebanks in order to bring him to the International Criminal Court to be tried for war crimes. Blaming himself for Caesar’s injuries, Ross disbands the Expendables, not wanting his team to get killed following him, and leaves for Las Vegas where he enlists retired mercenary-turned-recruiter Bonaparte to help him find a new team of younger mercenaries to pursue Stonebanks. The recruits include former U.S. Marine John Smilee, nightclub bouncer Luna, computer expert Thorn, and weapons expert Mars. Skilled sharpshooter Galgo (Banderas) asks to be included in the team, but Ross turns him down.
The new team members rendezvous with Ross’s rival Trench Mauser, returning a favor for Ross. Drummer has traced Stonebanks to Romania, where he is set to make an arms deal. Ross and the new recruits infiltrate an office building Stonebanks is using as a meeting place and, having to kill a few men in the process including arms buyer Goran Vata, manage to capture Stonebanks. In transit, Stonebanks begins to taunt Ross and explains why he betrayed The Expendables in the first place. Ross nearly kills him to shut him up but, despite Stonebanks egging him on, he stands down. Stonebanks’ men catch up to them, with the aid of his GPS Tracker, and fire a missile at the team’s van. Ross is thrown into a river, while Smilee, Luna, Thorn and Mars are captured by Stonebanks’ crew. Ross kills Stonebanks’ retrieval team and escapes.
Stonebanks sends Ross a video, challenging Ross to come after him and giving him his location in the country of Azmenistan. While preparing to leave and mount a rescue alone, Ross is found by Galgo, who offers his services again. Ross gives him a chance, later accompanied by the veteran Expendables. They rescue the young mercenaries, only to learn from Stonebanks that he has rigged the place with explosives. As both the young and veteran Expendables begin to fight one another, Ross convinces them to work together in order to take down Stonebanks once and for all. As the final battle begins, Thorn is able to use a jammer device to delay the countdown, giving them just under half an hour before detonation. Stonebanks then orders the armed forces of Azmenistan to attack the building with full force, including tanks and attack helicopters. Drummer and Trench arrive in a helicopter to help, alongside returning Expendables member Yin Yang.