GROUP BY a number of DAY in SQL Server
Add to your WHERE clause: AND DATEDIFF(DAY,'2019-02-09',TheDate) % 3 = 0

This will evaluate the number of days between your initial date and the current value of TheDate. The modulus (%) operator will return 0 for every 3 days in the timeline. Note that performance may take a hit evaluating this expression for every row, depending on the number of members in the dimension.