I honestly don't see the point of doing an IT: Chapter Three. Yes, Pennywise is cool, and the mythology detailed in the book is fantastic. BUT...
...exactly what kind of story are you going to tell? Watching IT crash to Earth in a meteor and feed on dinosaurs and eventually cave-men (er, sorry, I meant persons of the cavern-like persuasion, just in case there are any Social Justice Warriors reading this although I care not one whit about their feelings) and then... what? Pennywise never faced any challenge to IT's existence until The Losers. So we'd have an Antagonist with no challenge, hence no drama, hence no story to be told. Watching Pennywise torture and kill people over a span of generations will get dull real quick because it's an aimless journey. The mythology King wrote in the book was to serve the main story, which has already been told in both films. The best we can hope for now is the director will get his wish from the studio to do a "supercut," which is both films edited with previously unreleased footage and some new scenes that Muschietti said he wants to film, thereby fleshing out the story even more.
Of course, if we're still around in 27 years, the next logical step would be a slavish adaptation of the books in cable series format.