Complex sentence: After a short run on the Portuguese Liga they have been promoted three times to the division but have been relegated the following season but Feirense hasn't been able to get back up to the first division since its third relegation in the 1989/90 season.
Split into simpler sentences: After a short run on the Portuguese Liga, they have been promoted three times to the division. Unfortunately, they have been relegated the following season but Feirense hasn't been able to get back up to the first division since its third relegation in the 1989/90 season.