Still don't see the US reaching a limit of 3 runners though. And Europe is a continent full of small countries.
Different stat for you:
Tokyo 1500m final:
3 Brits, 1 American.
Tokyo 800m final:
1 Spaniard, 1 Brit, 1 French, 1 American.
Eugene 1500m final:
3 Spaniards, 2 Brits, 1 American.
Eugene 800m Semis:
2 Spaniards, 2 French, 2 Brits, 0 Americans.
In the major championships of the past 2 years the US had the same amount of finalists as Poland.