If you don't know by now, there will be a Ghosterbusters 3, and filming will start very soon. Word is they will be 4 new Ghostbusters in the film, no they are not tossing out the original guys. Only Bill murry is still holding out, not sure why but lets see what he has to say:
Well, even if Murray was there, the plan was that the originals would just be there for the sendoff of a new batch as they went into retirement, which doesn't require all of them to actually be present. Akroyd and Ramis wrote the scripts for both Ghost Busters video games a few years back as a prototype of how it would work.
Spoilers in black for those games:
The first game is set a couple years after Ghostbusters 2. Lewis was kicked off the team and is back to being their lawyer, and a second unnamed rookie had something bad enough happen to him that now Peter and Winston don't want to use real names with new rookies (actual quote, "No names, Ray. I don't want to get too attached this time. You remember what happened to the last guy."). They recruit a third rookie, who is with them through the game, which actually has a really good storyline tieing both of the movies into a single larger plot involving a system by which Evo Shandor plans to usurp Gozer and become the destroyer god himself. In the ending credits, he's offered a franchise deal in another city.
In Sanctum of Slime, the awful downloaded game, some years later that rookie is a member of a new team who's taking over the home office as the original four are preparing for retirement.
As a shameless plug, the first game is pretty good and can be had for like $8 on the 360 or PS3. Gameplay is sort of a DayGlo Gears of War, a lot like the Transformers games from Highmoon. Sanctum of Slime is awful and we should all agree it never existed.
As for Ghostbusters 3, like I said, with the limited scope that the original team is planned to have in it, Murray's absence won't be felt that hard. As for why he doesn't want to do it, he's a notorious primadonna and can be very difficult to work with, and he's said he came away from the video game with a bad taste in his mouth. Supposedly the director treated him like a child. In the, "Sit in the corner and read your lines or so help me I'll turn this car around and nobody gets icecream," sort of way that a lot of directors have said you need to take with him, but his clout as an actor makes most of them afraid to actually do.