I remember seeing a special about the POTA movies some years ago, and I'm pretty sure they said that
Battle was the only movie written with a sequel in mind. Certainly the ending of
Escape was meant to be ominous, to show that the seeds of the apocalyptic future had been sown, but as I said, I think the idea there was that it would be many generations down the road, not just a couple of decades as the next movie had it.