Hi,
Mmm. I'm not clear on what you want.
One solution would be to have 3 characteristics : small, medium and big.
Each one with values : 0, 1, 2, 3, 4, ....
I would recommend not going too big on the values.
Then, add the characteristics to the product, add all the combinations as variants, and for each variant enter the price you want to charge.
This will work fine up to a few hundred variants.
After that, the loading of the page will likely crash due to the amount of variants to load and process on the frontend.
So that means you'll be able to go up to 5+5+5 but not much more.
The advantage of this solution though is that you get the maximum flexibility possible as you can set a specific price for each combination without having to enter rules like -2€ for 2 products in the cart, -4€ for 3 products, etc, which apparently won't cut it in your case.