Multiple choices for dropdown

Hello,
Please forgive me if this has already been answered. I searched but I had a difficult time with what search phrase(s) to use.

How do I add variations when the same item is ordered more than once? Example: I give away a free t-shirt with each item. When an item is purchased, I ask for the size of the free shirt, small, medium or large. This works well.

But when a customer chooses to buy three of the same SKU, they only get one choice of shirt size. How can I offer a shirt size for each item purchased?