Marvel's Avi Arad was recently interviewed by iFMagazine. The Q&A with Arad included a handful of questions about the Spider-Man 3. One of his responses gives some insight on the villains from the upcoming movie:
iF: There are four villains this time, Venom, Sandman, and Green Goblin and when is the fourth going to be revealed?
ARAD: Right we have four villains. Pretty soon [we'll reveal the fourth villain] I believe. Maybe around Comic Con we'll reveal something and the madness will begin.
The entire iFMagazine article can be read HERE.