Hi @Elizabeth1844
I would just put the price assuming all 3 bedrooms will be used.
If a guest books a 3 bedroom house, and say only 2 people would come, I would be suspicious. Why not book something smaller and cheaper?.
On a personal note, I usually travel with friends or family. last time we were in Iceland, we booked 3 bedroom houses, as we were 1 couple and 2 single friends - I made a point to state that to the hosts, so they knew from start that I needed all beds made up and all bedrooms would be occupied.