The last time Newcastle averaged 15k or under was 1915 when we averaged 14,545. The last time Sunderland averaged 15k or under was 1989 when they averaged 14,878. The last ime Man City averaged 15k or under was 1965 when you averaged 14,753. As I've said before, the one thing very few clubs can have a go at Newcastle about is our crowds.
Newcastle have had 76,945,710 fans attend home games in the league since 1894, not including this season and have had an average of 15k or less 7 times, all but two before 1900.
Man City have had 72,404,583 fans through the gates in that same time period and have had the same number of average crowds less than 15k, 7 times and all but 1 of those was before 1900.
Sunderland have had 62,808,363 fans attend home games during that time, they've averaged 15k or less 17 times. They've averaged less than 15k 11 times since 1900.
I would say that all 3 clubs have been really well supported.