Not sure how they will split it between them, I'd assume that they will all take a pass at the story that Lucas had and that Arndt has fleshed out. Highly doubt they would completely do one each without seeking the advice of another, maybe taking it in turns for the first drafts or something. I like the idea of the three of them working together, they all bring their own qualities.
Kasdan's also been officially announced as a producer for all three.
