Well at the end of SM2, when Harry ( James Franco ) found his father's Green Goblin costume and waepons, I figured that they would somehow stay close to the comic and have Harry take over for his father as the Green Goblin. But it makes sense to take another comic book enemy in The HobGoblin and use him ifor Harry Becoming the Hobgoblin. They'll probably just have Harry make a variation of the costume. James Franco plays an excellent Harry Osborn. Very close to the comic book character.
In my opinion the other enemy that we will see is The Lizard. In the comic book Dr. Curt Conners becomes the Lizard and in the first SM we briefly are introduced to Dr. Conners and then in SM2 he has a much bigger role.
I've read that Venom will NOT be one of the enemies in SM3. But you never know.
My guess is The Hobgoblin and The Lizard.
Don Cardi