IF ATTR([Week End Date])<=[Min of Dates] and
datediff('week',ATTR([Week End Date]),[Min of Dates])<=3
That made it a valid formula but returned Null. I'm guessing making date an Attr effects the calculations?
Usually wrapping the non-aggregate field in an aggregating function -- ATTR(), SUM(), MIN(), AVG(), etc., -- fixes the mismatch.
And ATTR() is my first go-to fix.
But yes, this is a date, so try MIN() instead.