@Valerie314 a person like this is not the kind of guest you want to have as early guest.
Sadly, they apparently already have a confirmed reservation?
A guest who blames the host because the guest was too slow / indecisive is the sort of entitled person who would blame you for any problem, even ones they cause themselves. (Case in point.)
At least there is a handy excuse for you to use regarding the discount:
"I am sorry, but the AirBnB system does not allow me to extend the 'First 3 guests' discount to anyone but the first 3 guests who actually confirmed their reservations. I am not in control of these special discounts. If the price is more than you have budgeted for travel, I understand if you want to cancel your reservation or have AirBnB find you alternative accomodations.
If you do wish to travel here and stay with us, we are prepared to host you!"