And why did you assume that? The only reason First Frights and its sequel have been separated is because they're clearly set in another universe, because it doesn't fit with the other games or anything else.
Those gang pages are for all video game appearances from 1986 to 2005 (and maybe some of the online games after that, that aren't based on Be Cool and LEGO).
The cast signed up for three films. They must have made a third but are keeping in their archives.
Just because you sign on for three films doesn't mean you're going to make three films. Or just because any actor is signed on to do more, doesn't mean they secretly did, and the studio is just keeping it under wraps. It doesn't work like that.
They were contracted to make three films, so they would have to fulfill that contractual obligation if a third one entered production. The third one just wasn't made (Cartoon Network was involved, too, so maybe they had a say). Those contracts they signed may not have any bearing now, and the cast probably would have to sign new ones if they were to make a sequel. Hayley and Robbie have gone onto bigger things, while Nick and Kate have gone into the background.
There is no secret third film that they put time, effort and money into that Warner and CN just decided, "You know what? I think we're just going to shelve that film, instead." It's preposterous (as there's no proof), and it's just something you're fantasising.
Just think about. They are probably a lot of movies theatrical or direct-to-video that get made but never released.
Maybe. But those are probably known about. You're entitled to your own opinion, but it sounds like you're making a conspiracy theory about a Scooby-Doo sequel secretly being made. And, I'm sorry, but that just is not the case. Maybe one was written, but I think even that would be pushing it.