@Susan384 This may sound contentious (it's not and it's not mean to be), but why would you want to offer such a significant discount for a mere 3-day booking? If you want more 3-day (or longer) bookings, simply set your rates to what you want them to be and then set a 3-day minimum stay. Or, like we do, set the base minimum to 2 days, but then set custom minimums of 3 or even 4 day stays looking out 2-3 months in the future. That way, you'll fill the longer stays first (at a decent, but not heavily discounted rate) and then, as you reduce the min stay as the dates grow nearer, you can fill in the calendar with 2-day bookings. (At our place, we simply don't allow 1-day bookings because we don't want to clean that often.)
Reconsider your pricing and your various minimum stays. As others have said, AirBnB doesn't have any specific mechanism for discounts other than weekly or monthly.