I haven't seen any specifics but, as you say, the first three books are a natural trilogy and hollywood loves trilogies.
I do know Mark Strong is playing Matai Shang who by the canon shouldn't be seen until Gods Of Mars. Plus, it is Disney so I think it is a given the cast is already locked up for the next two chapters. Even better, from the corporate point of view, the rest of the series doesn't even need the same actors.
As much I love the first three novels, though, I have to give a slightly higher nod to Chessmen of Mars. Burroughs really cut loose on that one and snuck in a lot more of his usually ignored social commentary. Plus, my sympathies go to poor Gahan; as heroic a barsoomian as any, but trying to woo a princess who measures all men by the examples of her father and older brother.
