Aykroyd had an idea, had a rough draft, and then, supposedly, Ramis helped him finalize it. It was presented to Sony, who owned Columbia by then, and Sony rejected it, as Dan said, because they “wanted the next blair witch,” in the sense of low budget, big profit. And we all know that GB3 would not be low budget.
Reitman showed some interest early on, and offered to produce while Ramis offered to direct.
More recently, Dan Aykroyd declared GB3 dead because Bill Murray, who owns a fraction of the rights, (1/6 I think, can anyone confirm that?) didn't agree to a sequel, because one, he thought the character Venkman had been explored to it's fullest, and two, he didn't want another to be done EVEN if he wasn't in it.
Which poses an interesting question? Are these conflicting storylines? Or perhaps, as I'd like to think, did Sony finally Green-light the project and now it's just Murray holding them back?
Either way, Dan said it's done for.
But I like to take the viewpoint of a few people on this site, including Chad and Netsolo, in saying that I think we may see something happening in the next 5 or so years.
Who knows?