I've been doing a little bit of digging and here's what I've found...
Kirsten Dunst has made claims of who the villains where in the upcoming Spiderman movies and last time only one of them was actually in it (Dr. Octopus). So the chances are in Spiderman 3 only one of Venom or Sandman will be in it alongside the Green Goblin. Higher chance of it being the Sandman because the producer said a while back that Venom is one of his least favourite characters from the Spiderman comics.
The picture I posted up there also resembles the
Spider-Armour more than the
Symbiote.
Another theory: Topher Grace has been rumored to actually be playing the Chameleon and that the other multiple villians in the movie (both those confirmed and those unconfirmed) aren't real, they're all just the Chameleon taking on their forms. In some ways, it makes sense. There have been multiple people announced as cast in important roles, but who's roles haven't actually been revealed. Having several villians would weigh down the movie, and Sam Raimi has admitted he doesn't want to go that way, but if those several villians were actually all just the Chameleon...
Again, entirely just a rumor, but if it turns out to be true the "twist" that Topher Grace is multiple people is already spoiled.