I had the feeling season 1 was already written in a way that it could "end" the show:
Patrick Stewart is of old age. Being the main character of a new show is pretty demanding for your health. I'm pretty sure - if it would have been obvious Stewart didn't have the condition to continue, Picard's "death" in season 1 would have been final.
Since Stewart is a tough guy (and still hits the gym) they found out he's on board for another two seasons - so he got resurrected as a robot.
That means - it's an absolute possibility there will be a season 4. But I guess they won't make this decision now - they will make it in a year, depending on weather Stewart is still game by the time.
Until then, I suspect season 3 will again serve as an end/not-end, with a somewhat "proper" ending, that still allows to get back in action later on.
I don't know. I think it'll be pretty definitive. It won't exclude the idea of a potential fourth season, but it will feel very much like a closing door. The producers have been quite loud (relatively speaking) that the third season is it with the slight caveat that they'd do more if Stewart and Paramount were interested.