Exclude Dimension from Aggregate Calculation depending on Condition

Hi - hope to be able to explain this as best as I can as I cannot share the TWBX.

I'm showing a simple graph with a weekly trend of sales of a product line as a percentage of total sales for that week:

Week 10|Week 11|Week 12|Week 13 :-:|:-:|:-:|:-:| 83.5%|79.8%|81.3%|81.8%

These numbers are incorrect because (as part of the calculation of the percentage) they also include total sales for shops selling less than 3 units per week.

I would like to exclude from the percentages, the shops that sell less than 3 units of this product.

If I put a conditional filter on excluding shops that sell less than 3 units, Tableau takes this as a row total of 3 units and ignores the weekly aggregation that I am showing here.

Any help would be appreciated.