I think we'd all like to see a GB3 but here's the main problem: many ghostheads would be upset with the results.
Just like with SW Ep. I, many fans would be upset as a GB3 wouldn't be like the original.
I liked TPM personally, although many die hard SW fans hated it.
This is a guarantee. No matter what or how GB3 is made, tons of ghostheads wouldn't even like it.
I still think GB3 is a bankable idea for Sony, but that doesn't guarantee it's success in the eyes of ghostheads.
They can't duplicate the same formula unless Bill, Harold, Dan and Ernie are all still the main characters. And you know that won't happen (or probably won't). That's how GB3 would turn out. Even if it was good, it wouldn't be the same formula and many ghostheads wouldn't like it.
As for JP3, I personally believe that it was better than the 2nd but not as good as the first. That trilogy in itself is a quality trilogy.