In each of the three main Superhero Threequels, the Hero turns evil and becomes their own Villain. While all Villains are dark mirrors of the Hero in some way, the Hero literally becomes a dark version of himself. In Superman III, tainted Kryptonite makes Superman apathetic and selfish. Ultimately, the Kryptonite splits the evil Superman from the good Clark Kent and the two fight in a junk yard, with Clark besting his evil half in the end.
Similarly, Spider-Man 3 has Spider-Man finding the alien symbiote, which amplifies his aggressive and selfish tendencies. He acts like a general ass for most of the second act before finally deciding to get rid of the suit. In doing so, however, he creates Venom, which is as close as he comes to facing a dark version of himself.
In X-Men: The Last Stand, Jean Grey becomes the Dark Phoenix when the true potential of her powers are unleashed and she joins Magneto and the Brotherhood, leaving Wolverine to lead the fight against his one-time Damsel
In each of these movies, the Hero first revisits his past: Clark returns to Smallville, Peter learns the truth about his uncle’s murder, and Jean Grey returns to her childhood home. In every case, the past helps explain who the character is and triggers their turn to dark side. Clark finds himself torn between his hometown roots and his big city life, wondering whether he has become as self-absorbed as the rest of Metropolis. This struggle is mirrored in the separation of the two personalities, with Superman as the selfish, city-dweller and Clark as the good-hearted country boy. Peter revisits his anguish over the way Uncle Ben died and, fueled by the symbiote, finds himself once again out for revenge. Jean Grey, meanwhile, has been literally repressing her powers because it could be dangerous for her to reach her full potential. When those blocks are lifted, so too are her inhibitions and she becomes the Dark Phoenix.
It is interesting to note that while Venom and Dark Phoenix both have roots in their respective comics, their origins are changed significantly in the film version. The Venom symbiote was originally thought to be an alien costume Peter acquired during the Secret Wars. While the symbiote does make Peter more aggressive than usual, the ret-con with Uncle Ben’s murder is not present at all in the comics and was only added to the film. Similarly, the Dark Phoenix was a force of nature, not a part of Jean herself. These changes to such well-known story lines emphasize their importance and show how important it is for characters to return to their past in the Superhero Threequel: the changes come from within the character instead of being the result of an outside force.
The next common aspect of Superhero Threequels is a new Damsel. In Superman III, Clark rekindles his romance with Lana Lang, while Peter Parker has a short-lived fling with Gwen Stacy in Spider-Man 3. X-Men: The Last Stand takes a different approach to this, providing Secondary Hero Bobby with a new Damsel in the form of Kitty Pryde. Unlike the original Damsels, these Damsels represent the same aspect that the Hero has been hiding and ignoring. Lana represents Clark’s desire for a simple country life, Gwen becomes a physical representation of Peter’s dark persona, and Kitty represents Bobby’s desire to have a girlfriend he can actually hold and feel (with Kitty’s power of intangibility providing a nice touch of irony). In an interesting twist, Jean herself becomes Wolverine’s new Damsel by adopting a new personality. This dark persona is just as attractive to Wolverine and there are moments when he seems torn about his feelings toward his new enemy
And so, with the past resurfacing, a new Damsel to woo, and an oversized ego, the Hero becomes a dark, evil version of himself, ruthless and uncaring. Fortunately, he is always able to overcome his demons just in time, often when he sees himself in the eyes of the Damsel (new or old) and realizes how much he has changed. He sheds his dark skin just in time to take down the true Villain and become a real hero once more.