Complex sentence: The club would make three consecutive Grand Final appearances in 1988,1989 and 1989--90, winning the 1988 and 1989 Grand Finals, but were denied a historic "three-peat" when they lost the 1989--90 decider.
Split into simpler sentences: The club would make three consecutive Grand Final appearances in 1988,1989 and 1989--90. Marconi won the 1988 and 1989 Grand Finals, but in search of a historic 3-peat they would go down in the 1989--90 decider.