@Annick77 it is possible you might have missed your opportunity to offer the "3 first bookings discount."
There is no setting or page to activate a promotional discount once the chance to offer it has been rejected.
AirBnB may offer you the chance again, or it may not.
Sometimes AirBnB will give the host a chance to offer a special, but that is when the AirbnB system decides it is time.
There is no way that I know of for you, the host, to cause the AirBnB system to provide you the opportunity to make these special offers.
There should be something like a coupon booklet in your account where you can keep these offers, to use when appropriate.