3 days is the minimum to submit for analysis. However, 7 days is better since sleep usually fluctuates from day to day.
At least 7 days is especially recommended when you are trying to tackle a sleep issue that does not happen every night (e.g., long awake periods in the middle of the night, early rising), sleep varies a lot from day to day, or weekends are different from weekdays.
If you have tracked more sleep, include all the complete days when submitting! The more the merrier.
So why do we allow submitting just 3 days, if 7 provides a more accurate picture? Because moms and dads are already exhausted! We would rather you get the help you need based off of fewer days of sleep, than nothing at all.