Third time is hardly a charm. I guess what goes up much eventually come down. The Spider-Man franchise was experiencing a rarity that few previous movie franchises had faced before: the sequel was better than the original. This most notably was the case with the Terminator movies, and most recently with another comic book movie, X-Men. But just like those two other franchises, Spider-Man couldn't "pull a Seinfeld" and quit while he was ahead. Spider-Man joins the ranks of those trilogies by adding a third movie to the series that does not deliver.
Spider-Man had been following the "one movie, one villain" template in the previous two episodes. Those two movies were great. So I guess Sam Raimi fell into the same trap that bachelors fall into when they first learn to cook on their own. Most men in the early throws of bachelorhood will read the directions on the back of the Hamburger Helper box that say "cook for 20 minutes on medium heat." In lies the trap: they then think that if they cook it for 10 minutes on high heat, it will cook faster while maintaining some "cooking time ratio" they think exists (this is also the first time a bachelor learns how to disable a smoke alarm). Raimi falls for the movie equivalent of this trap by suggesting that if one villain in a movie is good, then surely three villains in a movie is triple good, right!? Wrong, sir! See exhibit A: every Batman movie after the first one. In fact Spider-Man 3 most parallels the third Batman movie, Batman Forever. Like Batman Forever, Spider-Man 3 is hokey edging on ridiculousness. Some scenes were outright cheesy with villains making agreements to kill Spider-Man like they were trading baseball cards and there is even a dance number! At least the dance number in Batman was relegated to Prince's music video and not made a major plot point like in Spider-Man 3. In the comics, the black costume made Peter Parker selfish, vengeful, cold-hearted, and overly aggressive. In the movie, the black (or really gray) suit turns Parker into the same type of person but not to the degree it should have. One thing the suit does in the movie that it didn't do in the comics was transform Peter Parker into some kind of silly, emo, dancing machine (complete with emo hair-do). I don't why they went in that direction. Something tells me Hollywood is trying too hard to appeal to tween-aged girls.
All in all, the script was not well thought out. It contains too many sub-plots and characters going off in so many directions that not even the best writer could wrangle them all in. On top of that, the villains are Sandman, who was never that interesting in the comics, and Venom, whose concept of being the evil version of the hero never results in quality writing, and New Goblin who isn't that strong due to the campy acting of James Franco. The movie is formulaic. Just like the first two, it begins with the main villain being created through a scientific accident. And again just like the first two, it ends with a kidnapped Mary Jane hanging on to something while Spider-man battles the villains. Just a thought but why doesn't some investigative reporter in New York put it together that Mary Jane is the one always getting kidnapped. Hey journalists! You think she might mean something to Spider-Man!? 'Nuff Said!