@Megan940 You can't add extra charges that weren't mentioned when the guest booked, and no one is ever going to pay you for something after they check out.
If I were you, if there is any way you can cancel this booking without penalties, I would. There is no way you should be accepting a 3 month booking as new hosts. A guest ceases to be a guest after 30 days- they will be classified as a tenant and covered by local landlord/tenant laws. Airbnb provides none of the safeguards landlords use with tenants.