Discount based on Cart Variant

[jbrennan]

I am using cart variants so a customer can choose their size.

I would like to apply a small discount if the customer orders a quantity of three or more of the largest size. I can use a Tax & Calculation Rule to apply a discount based on quantity but I can't seem to find a way to restrict the discount to a specific cart variant.

Is it possible with the default Virtuemart tools, if so how do I accomplish this or do I need a third party extension?

Thank you in advance for your help.