Hello @Mariana393,
The way Airbnb operates there is no facility to automatically charge per room. There is only, as you have set, the facility to charge per person.
Airbnb's system is that the price you set, with any added per guest charge, is for the entire place i.e. all 3 rooms in your case.
It is possible to charge per room but you have to
1) mention this in your description, house rules and messages to guests
2) try to get it from the guests after they've booked but before checkin by using Airbnb's resolution centre
Airbnb's policy does not allow collecting money outside of the aribnb platform.
Guests who have a confirmed reservation can't be charged any more than they've already paid Airbnb if those charges were not noted on the listing when they booked.
Steve.