@Jessica1531
You have 2 listings active.
I assume your question is about this one: https://www.airbnb.com/rooms/38439032
Max amount of guest is set to 2 and there is no extra charge set for the second person.
If a third person is staying, it can not be charged within the booking form of listing, unless you change max. of guests to 3 (and mention also sleeping facilities for third person) AND add the extra fee for additional guests after the 2nd guest.
After that you can try to change the reservation ("change" option)
Otherwise you need to charge the guest for the extra money via resolution centre.
Hope this helps,
Emiel