Shipping Problem with quantity

Hello,

I have a problem because I defined the fixed rate shipping costs at 3.00€ and a extra shipping cost on a product of 1.00€ .
For 1 product, ok, the shipping cost in the checkout is 4.00€ but when we have 3 same product in the cart, the shipping cost is always 4.00€ and not 12.00€ (or 6.00€)

How to have a shipping cost depending on the quantity of product ? (I saw the addon for a shipping cost with total value step, but not with quantity directly ..)

Thank you in advance ! and sorry for the -approx- English ??

Good day !
Axel