Dan Aykroyd explains that he has been co-writing the Ghostbusters 3 screenplay with Eisenberg and Gene Stupnitsky:

“He was talking about the writers from Year One, and I think he was reacting to the box-office success and the general public view of the film, which in my view was a very serviceable comedy, and in the end I think they’ll make their money back,”

“I think he was concerned that the writing on Ghostbusters 3 by these guys would not be up to standard, but I can tell you firsthand, I’m working on the script now and those two—Stupnitsky and Eisenberg, [writer-producers of The Office]—wrote Bill the comic role of a lifetime, and the new Ghostbusters and the old are all well represented in it…we have a strong first draft that Harold [Ramis] and I will take back, and I’m very excited about working on it.”

Read more here: http://www.vanityfair.com/online/daily/2010/10/dan-akroyd-writing-ghostbusters-3-script-selling-vodka-out-of-his-rv.html