US to require COVID Testing for International Flyers

I do not understand how the 3 day rule will work. If you go for a test 3 days before your flight the results may not be ready. Same for 2 days before flight
If you have the test 4 days before the flight it will be outdated. The rule should be a max of 3 days from when you received the results. Canada will only accept the nasal swab. No rapid tests. The USA has not published the full document describing the regulations .