Plot
Twenty-two months after the events of the second film, Ellie is pregnant, making Manny desperate to make life safe for both her and their upcoming child, not wanting them to meet the same fate as his previous wife and child. However, Manny's desperation alienates Diego, who contemplates leaving the herd, feeling like he's losing his hunter instincts. On the other hand, Sid becomes anxious about being abandoned, leading him to discover three apparently abandoned eggs underground that he decides to adopt. Manny tells Sid to return the eggs, but Sid ignores him and looks after them, with the eggs hatching into baby Tyrannosaurus the following day.
Although Sid tries his best to raise the dinosaurs, their rambunctious behavior scares away the younger animals and destroys the playground Manny built for his child, angering him. The mother Tyrannosaurus, whose eggs Sid took, arrives. When Sid refuses to return her children, she carries both Sid and her children underground. The two mammoths and possums follow them, discovering that the icy cave leads to a vast subterranean lost world populated by dinosaurs. After escaping a territorial Ankylosaurus and reuniting with Diego, they are surrounded by more dinosaurs, but are saved by a one-eyed weasel named Buck.
Buck reveals he has lived in the jungle for a long time, and is trying to hunt down Rudy, a monstrous albino Baryonyx who took Buck's missing eye. He agrees to lead the group through the jungle to Lava Falls, where the mother dinosaur takes Sid and her babies. Meanwhile, the mother Tyrannosaurus tries to get rid of Sid, but, after several failed attempts to do so, slowly grows attached to him. The following day, however, Sid is separated and chased by Rudy. Sid escapes, but gets stranded on a loose rock floating on a river of lava, heading to plummet over the falls.
Nearing Lava Falls, Ellie enters labor, and a pack of Guanlong causes a rock slide that separates her from the rest. Buck commands Manny and Diego to stay behind to protect Ellie while he and the possums rescue Sid. Diego regains his edge from defeating the Guanlong while supporting Ellie emotionally as she gives birth. Manny slows down the rest as he makes his way up to her. Buck, Crash, and Eddie ride a Harpactognathus to Lava Falls, but a pack of Quetzalcoatlus follow, preying on the possums and forcing them to detour through a canyon. They defeat the Quetzalcoatlus, and return to Lava Falls just in time to rescue Sid. Back at the plates, the remaining Guanlong are defeated with Manny reaching Ellie just in time to see his newborn daughter, agreeing to name the baby "Peaches". Sid is happy to reunite with his friends, but is sad that he never had a chance to say goodbye to the Tyrannosaurus.
Before leaving the jungle, they are ambushed by Rudy, but are saved by the mother Tyrannosaurus, who charges at Rudy and knocks him off a cliff. Sid then says goodbye to the dinosaurs, and Buck, now without a purpose in life since Rudy is gone, is offered to join the herd and live on the surface. However, a distant roar tells him that Rudy is still alive. Changing his mind, he sends the herd home and blocks off the path to the lost world. Manny and Ellie welcome Peaches into their frozen world, and Diego decides to remain with the herd, while Buck stays underground, trying to tame Rudy.
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