@Debbie10 From what you have described, there was no cancellation. The fact that the guest altered their reservation , with your approval, from 3 days to 1 day does not constitute a cancellation. It's an amended reservation. Your guest stayed that one night and that was that.
If the guest had arrived, say, and spent 1 night and then cancelled the remaining days, then you would have been compensated under your cancellation policy. But that's not what happened. They changed (not cancelled) their reservation. If you had declined the change in the reservation and the guest then cancelled then you would have been compensated.
I can certainly understand why you think that the change in the number of days from 3 to 1 meant the guest cancelled 2 days. But they didn't in so far as Airbnb is concerned. They amended the reservation to 1 day which you agreed to so it was a new contract between you and the guest. Can you see the difference?
In the future, if you want to be compensated for a reduction in the number of days of an original reservation, don't agree to a lessor number of days as that changes the terms of the reservation.