i was reading an article on tha net about dan akroyd talking about gb3 not being made it was pretty old but heres wut he said "It doesn't look like it's going to happen for the same reason they aren't going to make Men In Black 2. The cost is too excessive for the studio to see it to be economically feasible. It is a shame too because everyone wants to do it.
this was in 99 and since they did make mib2 theres a good chance they'll make gb3 so lets just hope

Please read old posts. There are a lot of old posts with that same information, all you have to do is look in the gb3 forum. Here's one:
http://ghostbusters.net/disscussion/ghn/3/43516/

Well, gee, didn't they do MIB2? If there's enough response to a movie, a sequel is almost inevitable. Surely Columbia Pictures knows about all us fans?