The new comic is coming out this fall, which leads me to believe that a GB3 is very much possible. If the comic is popular enough and there is enough demand for a new GB movie, it may actually happen. There have been a lot of rumors since a new GB installment was talked about in 1996. But Dan Ackroyd seemed interested. But there are a few problems with my theory. One it that the original actors (Ackroyd, Murray, Ramis, Hudson, Potts, Moranis, Weaver) have gotten older and may not be interested. Plus, they have gained some more weight (Ackroyd, Ramis)
This all means that Sony doesn't want to waste money (I don't think they would be) on a new GB movie and risk looking like fools if it flops. Another problem is how are they going to come up with a good enough script to persuade Sony. As you can see, there are more problems than sure-fire answers. But only thing that can really make the chances better, are buying the comic and demanding a new movie whether it is on the internet (via website or chat room) or on the streets. Some people are claiming that GB is dying and people are going to forget about it. (Harry Potter will be forgotten) That's not true. Trust me, one way or another, a GB3 WILL be made.