I've a request to add a product and give a 10% discount on 3 or more. The issue is that the product is a a tool that comes in 5 different sizes and people will not buy multiples of the same size, but one of each size.

I'm stumped on how to set this up. If I make each size tool a separate product, then the best I can understand there is no way to offer a quantity discount since it would need to apply to multiple products.

If I set it up as one product but with each size as an attribute checkbox that adds $xx to the price, I can't figure out how to set the discount for 3 or more.

I'm using 1.3.8.