@Pete69 This is an example of Airbnb's confusing, incomplete, and ambiguous wording. By flexible check-in time, I assume you mean your listing allows guests to check in anytime on the day their booking starts, or anytime after a set time.
I also use moderate and have an 11am-11pm check-in window.
Just because you have flexible check-in time, check-in day is still check-in day, not 3 days later.
With moderate, if they never show and cancel 3 days into a 7 night booking, it seems they should be charged for the entire booking, as they didn't cancel before check-in. But Airbnb might refund them all the nights after the first 3, when they cancelled.
Never having faced this situation, I can't say for sure.