The question is who will be the studio animating and producing it if the the rumors and speculation of Wit Studio being done after the 3rd season are true. *cough*Production I.G */cough*
The manga isn't a problem as it's in the final arc currently and will most likely be completed before the final season. I was talking about the studio that will do it.I'm pretty sure they can get the rest of the manga into this final season.