Here's the thing. I would agree there would be no need to develop more, if in the end of Spidey2, when he finds out about Peter being Spider-man, he accepts his destiny to avenge his father. But the thing is that he didn't. He was extremely conflicted about, to the point where he disobeys his ghost father's direct order:
Norman: AVENGE ME!
Harry: NOOO! (throws knife at mirror)
Then, he finds out his father was the Goblin. Wouldn't Harry at least consider, hey, whoa, my pop was a nut? No, we end with that cliffhanger, and boom, in Part 3, Harry is new Goblin. That's the WTH part.
So are we to assume that behind he scenes, he goes through this conflict and resolves it? See, that's what I wanted to see. This should be like Star Wars Episode III for Harry. Him embracing the Goblin identity in all it's psychotic glory.
Some of you say, "well, it's already too long". My response to that is, well, then cut out the crap in the movie. Like the Sandman being Uncle Ben's killer subplot. Or having 3 too many villains in the movie.