You haven't seen Spider-Man 3 yet? It's skippable. You really want to go? Try a matinee. As I said to D on Friday, it's not so much about "what happens next" as it is "some stuff happens." She called it a ride, which is also a good description. You don't really need to pay attention to the story, because nothing of real consequence happens; just enjoy it while it's happening. There's action, there's comedy, there's eye candy. Not much else.
I believe there are two major axes on which superhero movies can be charted: exposition and tone. Some flicks try to explain everything, and others just go for the fun. Too much exposition about how the Phantom Zone actually works will bog down your story; not enough exposition and your audience won't be able to suspend their disbelief. Take yourself too seriously, and people will get thrown out of the story; make everything into a joke, and they won't be able to engage in the first place. It's a delicate balance.
Spider-Man 3 falls somewhere in the bottom two quadrants: not quite enough exposition, especially in the Venom department, and with wild tonal shifts that undercut what the teasers and trailers were selling as a darker story than the first two movies. And even if there was a justification for each of the three villains to appear in this story, there wasn't a good reason to overstuff the movie.