Complex sentence: Even into the earliest two seasons of the 1980s, the club were performing respectably with a third-place finish in 1980--81, however by 1983 they had slipped dramatically and were involved in relegation battles.
Split into simpler sentences: Even into the earliest two seasons of the 1980s, the club were performing respectably with a third-place finish in 1980--81. However, by 1983, they had slipped dramatically and were involved in relegation battles.