When product is selected, the slide cart has the 3 variant (product) options to select from.
Example.. Buy 2 Deodorants Get 1 Free Product, once selected the cart drawer opens and has Deodorant 1, Deodorant 2, Deodorant 3 options, then it puts all three into cart and 1
But the flow might be a little different from what you expect: customers add 2 Deodorants in their cart, then they will see a gift slider offering the third Deodorants for FREE. And customers can choose their preferred variants.
You can see my screenshot, this is how a slider may look on the store