I'm not sure I understand... Do you only sell 3 products?
So if they add just the first product, then they get just 13% off that product regardless of the quantity of that product?
Is that the quantity of each product or the total of all products?
Are the percentages for each product? or is this a volume discount for each product? So 13% off the first one, 22% off with 2 and 40% off for 3 or more?
I need to understand how you want your discounts to work before trying to describe how to implement them.
Do all of your products have the same discount schedule?