Dan Aykroyd went so far as to give out casting information, Sigourney Weaver gave out confusing clues, and Bill Murray went back and forth on his opinions of GB3. Even Harold Ramis got in on the rumor telling fun. Turns out, its all horse shit. Ivan Reitman, producer of Ghostbusters I and II, spoke on the subject while doing press for his latest non-Ghostbusters movie.
"There is nothing... I mean, all these stories... there has been a ton of stuff about casting, about who's in... none of it is true."
Bill Murray just got the script and probably hasn't even read it yet. Reitman confirmed that the script includes all the original cast members and went on to say "I hope we get to do it".
So...either there is no real news on GB3 yet and all the other cast members are yankin our chains, or Ivan Reitman is full of it. Its hard to believe that NOTHING that Aykroyd, Murray, Weaver, and Murray have said have any ounce of truth, but at this point who really knows.
Read more of the interview at ComingSoon.net.