accomplish apply automatically cart combination customer discount don ideally included month product products promotion support tip

We are doing a month long promotion which will discount several of our products (25), but not all of them.

The idea is to do a 3 for $12 kind of thing. Ideally, I'd like the discount to apply automatically when three of the products that are included in the promotion are in the customer's cart.

Does anyone have a tip on how to do accomplish this?

We can support a discount for "3 or more of the same product", or "any combination of 3 or more of any/all products", but we don't have any way to set up a discount for "any combination of 3 or more of just certain products (but not any/all products)".