While Dan Aykroyd was quoted in February saying that Bill Murray would not be involved with Ghostbusters 3, Murray himself revealed earlier this month in an interview with WGN (via The Playlist) that it's still a "possibility."
While Murray quickly changes the subject and talks about Wes Anderson's Moonrise Kingdom, he doesn't say outright that he won't do the third installment.
Aykroyd previously said that the film was in "suspended animation. 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."