Apparently Brett Goldstein, who plays Roy Kent, also recently hinted that he doesn't like to get involved in projects with no planned end point. So yeah, it's pretty safe to say they've planned it as 3 seasons from the start. This is good in that it won't go on too long, losing quality in later seasons like so many other shows.Bill Lawrence has said that from Jason’s POV the *only* way they’d extend it is if production happened down the road from his house. In other words, it was written to be 3 seasons and Jason doesn’t want to have to travel anymore to shoot it (which is his right).
The good news is that it holds up remarkably well on rewatching, so those of us who want more will have to be satisfied with that.