Answer: 3.
Since there were 5 teams in the league there were a total of 10 possible games (either way around):
Longton vs Fenton
Longton vs Blurton
Longton vs Hulton
Longton vs Fulton
Fenton vs Blurton
Fenton vs Hulton
Fenton vs Fulton
Blurton vs Hulton
Blurton vs Fulton
Hulton vs Fulton
Since each game distributed 2 points to the two teams there were a total of 20 points available to the league.
The results shown add to 17, leaving the remaining 3 to Fulton.