Murray recently called the idea of a Ghostbusters III "a crock" and told GQ:
"Harold Ramis said, Oh, I’ve got these guys, they write on The Office, and they’re really funny. They’re going to write the next Ghostbusters. And they had just written this movie that he had directed. Year One. Well, I never went to see Year One, but people who did, including other Ghostbusters, said it was one of the worst things they had ever seen in their lives. So that dream just vaporized. That was gone."
It looks to me like this project is DOA, and not just because of Murray's lack of interest. It appears that the creative elements of this movie are moving in a lot of different directions and aren't really communicating. And that's never a good sign when you're working on a collaborative project.