Thanks to STYD for the info:
Dan Aykroyd talked about this vaguely earlier this month, but he has now confirmed to The Telegraph that Bill Murray won't be a part of Ghostbusters 3, if the film gets made at Sony Pictures.
"I honestly don't know. At this point it's in suspended animation," he said when asked about the film's status. "The studio, the director Ivan Reitman and Harold Ramis feel there must be a way to do it, but Bill Murray will not do the movie. He doesn't want to be involved. He's got six kids, houses all over America. He golfs in these tournaments where they pay him to turn up and have a laugh. He's into this life and living it. I know we'd have a lot of fun [but] I can't be mad at him. He’s a friend first, a colleague second. We have a deep personal relationship that transcends business and he doesn’t want to know."