Superhero Sequel Weakness

What is wrong with "3" sequels, specifically action and superhero flicks? I can imagine the Hollywood meetings where the "3" superhero movies are planned:

"No, introduce another villain! Complicate this hero's relationships! Kill characters! Faster! More ridiculous! More unbelievable!"

I had the privilege last weekend of watching two threequels I had previously missed: Spider-man 3 and X-men: The Last Stand. Both movies were thoroughly entertaining, and had one thing in common: everything was a little bigger, a little more unbelievable, and a little more bewildering.

Threequels obliterate where the previous movies merely knocked things around. They mine off the original movies, carrying familiar plot elements from the previous movies to impossible proportions. As these unbelievable webs unfold, familiar characters begin to act. . . unfamiliar. One notable exception is the Harry Potter movies, which mercilessly forgot this tradition until the fourth movie, where Dumbledore loses his grandfatherly tendencies and all-but-strangles Harry in one uncomfortable scene.

Why do threequels do this? Maybe there's an unconscious "trilogy" mentality in Hollywood. Both Star Wars trilogies saved their biggest (and, in the new trilogy's case, dumbest) action sequences for last. Back to the Future 3 brought the series full-circle, nearly recreating the pivotal high school scene from the first movie.

Why? Why can't the third films be as subtle, charming and delicious as the others? Why must everything be caffeinated and fed through the filter of the Summer Blockbuster, even where the previous movies have avoided this tendency? Spider-man 3 alone jacked the evil twin concept, undermined the original film by changing story for the sake of concocting character motivation and introduced characters from the rafters to reveal important plot points to the audience when the movie couldn't figure out how to do it on its own. Every character and villain had so many bloody motivations that the movie became a swirling roller-coaster and lost track of its human elements. Three - count 'em, THREE - villains, and not one of them ever seemed to be acting like a human being. (To be fair, Green Goblin Jr. got a nice send-off, but his deathbed repentance was too little, too late. Not quite your friendly neighborhood train wreck - more of a technicolor hurricane.)

And X-men 3 was. . . directed by Brett Ratner. Which explains as much.

In X-men, every character was angry, all the time. The character whose death we mourned in the last film returns to kill everybody and, in rare moments of self-control pleads to be killed. . . again. Tasteful, movie. Professor X loses his poise and yells at everybody. Magneto seems hungrier and more harebrained than ever, and the previously-entertaining turncoat villain Pyro turns into. . . Draco Malfoy. In the rush to throw more on the screen, the movie creates a bunch of gimmicky mutants and can't find anything to do with them. The characters travel from scene to scene, not because people in their situation would act as they are doing, but because the script requires it. Plus, everybody dies (though the movie doesn't have the courage to keep these characters dead and/or powerless, if I understood the final Magneto scene and puzzling post-credits Professor X scenes correctly. Kudos, on the other hand, to the Spider-man team; I doubt Green Goblin Jr. will be back. Bless Raimi's soul, but James Franco was creepy in the third movie).

Both "3" movies had such a legacy going for them that they would have been nearly impossible to ruin, but neither had the meat of the previous films. When the inevitable fourth installments come around, will there be anything left to destroy?