How can you assemble all 6 villains by the third film? I'd really want to see the villains established before they're brought together. Common enemies that put them all away behind bars.
The idea is that some of the villains, whether it is Rhino and/or Electro, along with the Goblin, could be established during the second film. Even Adrien Toomes is suspected to be in TASM2, so the foundation, at a minimu, is there for the Vulture. The Black Cat or Felicia, is also rumored to be in TASM2, and she could start as "somewhat on the baddie's side".
Then in the third film you could take the leader (Goblin) who was already established in TASM2, have him break out one or two from prison (Ravencroft, which is in TASM2), have him hire/make/find the rest (which could be any of the list of possibles, including Mysterio, Doc Ock, Shocker, Jackal, Kraven, Chamelion, etc.
So the third film would be about the forming of the Sinister Six, Peter dealing with Gwen's death, MJ being introduced as a friend (not a love interest yet), and more on the Richard Parker story. In fact, in regard to the Richard Parker story the Goblin could (not saying likely, but possible) manipulate the Lizard to being one of the Six. That wouldn't mean that the Lizard would fight Spiderman, just that he might be forced to work for Osborn/Goblin.
Then in 4 you would conclude everything, with the finale being the ultimate fight. If Black Cat was one of the Six, she would switch sides (similar to what Catwoman did in Rises). And if the Lizard was being forced/manipulated to be part of the Six, he could realize the error of his ways, maybe take out one of the other Six, and slink away.
This really solves all of the concerns that people have had, including:
1. The Sinister Six is too much to do in one movie. (Instead it would be over two/three movies).
2. There will be too many bad guys. (Well, if Black Cat and/or Lizard were part of the Six, then it would really be 4 or 5 baddies. And the Cat and/or Lizard would help take down the hench baddies (Rhino, Kraven, Electro, etc.). For some reason I think I would really enjoy seeing the Black Cat soundly beat the Rhino...
3. It would be too fast to have Peter develop a relationship with MJ. (Well, this wouldn't happen until the 4th movie. During the third we would get to know MJ, but Peter would still be dealing with the loss of Gwen (which would happen at the end of 2). Maybe MJ helps him deal with the loss. Black Cat/Felicia could come into it as well, causing a bit of a competitive love interest, or at least flirtation. This could be used to get MJ to realize her feelings for Peter. In fact, it's possible that Peter and MJ will not be together in any of these 4, with just the hope of what it will be being at the end of 4.