Could you please tell more about your meaning of "Last N Weeks"?
Does it translate to a N*7 days from TODAY() moving window,
or may be it does mean three particular Week numbers including the current one?
The answer (and the view complexity) could be different depending on the definition.
For anyone to help you with the solution,
please, do not hesitate to post your try (as a workbook).
It could ease answering a lot.
And please, don't worry for it to be a total miss.
We're all learning from each other here.