Ghostbusters III is a go and will be out sometime 2012. IMDb has already posted the actors that will appear in the movie . Of course Bill Murray . Dan Aykaroyd , Harold Ramis , Ernie Hudson are back for their original roles and it is rumored that also Rick Moranis is also in on it. Dan Aykroyd and Harold Ramis will be the executive producers along with Lee Eisenberg , Ivan Reitman, Gene Stupnitsky as producers. Lee Eisenberg, Gene Stupnitsky are also in on the as writers of the screen play. The rest of this story I got from Wiki which is a better detailed description of the movie and some interesting background and history of the making of Ghostbusters 3.
Story and history of Ghostbusters 3 from Wikipedea
During the 1990s, Dan Aykroyd's concept for Ghostbusters III, under various working titles, revolved around "Ghostbusters in Hell", the idea being the characters could switch to a negative version of Manhattan called Manhellton. "We go to the hell side of Manhattan, downtown, Foley Square. It's all where the cops are--they are all blue minotaurs. Central Park is this huge peat mine with green demons there, surrounded by black onyx thousand-foot high apartment buildings with classic red devils, very wealthy. We go and visit a Donald Trump-like character who is Mr. Sifler. Luke Sifler. Lu-cifer. So we meet the devil inside." IGN described the script as being "too technical", with a new team of Ghostbusters that were not clearly delineated and had no group conflict or leading personalities. In 2002, Aykroyd said the film would be animated. Bill Murray has been thought to have held up the making of a third Ghostbusters movie, despite Aykroyd's enthusiasm, due to his preference that the movie be animated and his dislike of sequels. Harold Ramis mentioned in 2005 that he wanted Ben Stiller to join the cast, and the plot was to follow the three Ghostbusters attempting to find a replacement for Venkman while dealing with ghosts rejected from hell.
On September 4, 2008, Columbia Pictures signed on Lee Eisenberg and Gene Stupnitsky (writers for the U.S. version of The Office) to script a third film. The story will feature old and new Ghostbusters.Ramis had collaborated with Eisenberg and Stupnitsky on Year One. Aykroyd revealed Murray, Weaver and Ernie Hudson will reprise their roles. Murray agreed to reprise the part if Hudson's was expanded to equal the other three leads. Ramis said in June 2009, "I know all the guys want to come back except for [Rick] Moranis, who's disappeared. Not disappeared physically. But he's not too interested in entertainment at this time". Filming may begin in late 2009. Aykroyd also revealed there would be numerous Ghostbuster cadets in the sequel, and suggested Eliza Dushku (a friend of his who has "great comedy chops" and "might be a good villainess") for a role, or Alyssa Milano (who plays Dr. Ilyssa Selwyn in the 2009 video game). Ramis added that Milano would not be the only aspect of the game that may be in the film, telling an interviewer his idea for where Egon would be in the 21st century: "He's been working in the International Institute for Imaginary Science in Geneva, [having] evolved a post-rational non-conclusive logic for dealing with problems of chaos .. He's gone so abstract, even he doesn't know what he's been doing!" Aykroyd has said that Ghostbusters: The Video Game could be the closest thing to the third Ghostbusters movie but still plans on making the third film. He was also interviewed in the June 2009 issue of UK Official Xbox Magazine in which he revealed that "Because we’re talking about a third Ghostbusters film and the script is being written, I have very clear ideas about where Egon is. The only hint I’ll give you is that he’s moved past the physical plane of reality – he’s almost entirely on the ethereal plane."
In June 2009, Sigourney Weaver stated that she doesn't expect to be in the third Ghostbusters movie. Speaking to scifisquad.com, the actress said "No, I don't expect to have anything to do with it, although I wish them well. I hope my character's son, Oscar is one of the ghostbusters in the new film, though".