Create a free item in Woocommerce for each Offer you want to set up (setting price to 0: THIS IS IMPORTANT. Otherwise Woocommerce will give you ‘item unavailable’).
Under ‘Inventory’, check SOLD INDIVIDUALLY. This will disable users to increase freebie quantity once it has been added to the cart.
If you want to offer the same product on different offers, it is recommended to clone up the free item so that Woocommerce manage them as differente items.
Read HERE how to create a free item.