The following may be either an "oops" moment during an interview, or just misdirection, but the thing is that it actually makes sense.
Tim Phoenix is a special effects guy who did work on Spiderman 2, and is currently working on Where the Wild Things Are, and the Pirates of the Caribbean sequel. During an interview with a local paper in which he talked about how fortunate he feels with all the big projects he's been fortunate to be involved in he apparently let slip that he was working on conceptual designs for the upcoming Spidey flick.
Man-Wolf and Venom.
Now here's the thing... the previous two films have featured (thankfully!) just one villain each, so two villains would not seem to fit the mold of the first two films. However... Venom is a completely logical choice for a villain in this third film. Fans have been clamoring for it and personally, I think it would rock.
For now file this as "65% internet rumour", but man, it sounds good, don't it?
Source: Press Sun & Bulletin