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In short, Rebel Moon as a franchise is a success, judging by what we know so far. And Netflix obviously wants to strike while the iron is hot, which it chose to do by giving the green light to Rebel Moon: Part Three. Snyder himself confirmed this in an interview with Forbes where he said the following:
"We have definitely been working on a Part Three, as far as the story goes. We definitely know where we're all headed - we've known that for quite a while, to be honest. So yeah, I'm excited to make some more Rebel Moon movies - that would be fun."