Despite the rumors to the contrary, No one has signed up for Ghostbusters 3 yet. It had been rumored that the entire previous cast (Minus Sigourney Weaver and Rick Moranis) had already signed up and were extremely eager to get started shooting the new movie. The project was on hold as the studios were trying to convince Sigourney Weaver to come on board, and they were presumably still trying to find Rick Moranis, as he has been missing in action for some time now.

Apparently every single on of those "Facts" was nothing more then rumor.
"I wrote the story for the new movie with them, and Dan and I've been consulting. We're just waiting to see a first draft and where we are," Ramis told us, although he's very optimistic it will happen with Bill Murray and Ernie Hudson returning as well. "Everyone says they'll do it, they've all said they'll do it. No one has signed anything yet--we haven't signed anything either--but there's the spirit of willingness in the air."
http://www.comingsoon.net/news/movienews.php?id=56130

So the cast is eager to get on board, no one has signed yet. Well Hollywood being Hollywood, this typically means that everyone is waiting to see if the other person is going to sign up first.