Thanks again Mark, for your inspiration. I ended up with the following (complex) solution:
I added another Date column and created multiple automations:
When Status changes to In Review set Due Date to today and push Due Date by 21 Days
When Due Date arrives and Status is In Review notify Person
By first setting the due date to today I make sure that the notification always occurs in 21 days and pushing the date does not stack.
Secondly by connecting the notification to the status, passing the due date will not matter as long as the project is still WIP or in revision.
Thirdly no matter whether the due date had been passed, by setting the due date to today and then pushing the date when changing the status I make sure that I still have three weeks to go.