Did they know for months? Jacobs snowed the fans.
There's slowly appearing clues that the show WAS cancelled months age and everyone mostly kept quiet not to hurt the new episodes airings.
For instance, why do an hour show celebrating the fans in the tail end of the 3nd season? Those are two less half hour of scripted episodes. Plus the last two episodes "16" and "Goodbye" - this wasn't Jacobs heading bets, it was a producer who knew he wasn't getting a 4th year.
Sabrina booking tour dates months in advance when recording of a 4th season would have taken place.
Ryder's comments this week.
Disney's plan was 3 and out- 65 to 75 episodes to have enough to rerun for a few years. 30 episodes for Season 2 was part of this plan so they wouldn't need a 4th year.