How to manage inventory for specific product variants in Shopify?

Hi

I am building a new site and one of my products is a popcorn tin where the customer can pick the tin style, and then three different flavors. I need to set it up so that I can keep inventory for each of the tin styles, but not the popcorn flavors. I did this in a different ecommerce software by assigning skus to the tin styles but not the the other options (the flavors). How can I do this in Shopify?

Thanks

Ken