Combine 3 tables into a final result

Hi,
I have three sheets (one per month) in which are the clients and the products sold for the corresponding month. Every month new clients and products can appear. How can i calculate after 3 months how much each one sold (the result should be displayed in the 4th sheet).I have attached a file similar to what I have. Somewhere along the way I got lost looking for something similar on this wonderful forum. Someone can guide me in the right direction?
Thank you!

Hi sejur,

Please edit the question in your thread to "Combine 3 tables into a final result". Please do this first so we can find your question later.

Then - Excel works better with tables of data like shown in this example. I've used a Power Query Unpivot to convert your data to a better format. Then appended each to a combined table on the last sheet. I think you want a pivot table to show a total. I hope this is closer to what you want to see and ask about. Change the way you collect your data or learn about PQ and unpivot and append tables.
PQ UnPivot Append Pivot for sejur.xlsx