I think Spiderman 3 is going to work the same way. I doubt any of the three villains are connected. It's not like Batman 3 or 4 where they were working together. I wouldn't be stunned if they barely overlappy; as in Parker defeats Hobgoblin at the start of the film, Sandman is the focus of the final act, and Venom doesn't appear until the very end (in a sort of cliffhanger).

I'm a fan of Tobey as Spiderman and although I've never thought much of Dunst, I think she fits better than some smoking hot girl. The fact that she's Parker's girl makes it a little more believable. Peter Parker can cozy up to Dunst whereas Bruce Wayne should be doing better than Katie Holmes.