@Kenny134 I share my experience with long-term guests with you.
1. If you want a guest who is to stay for three month to book month to month, Airbnb cannot do anything for you. You may need to discuss with the guest about it. If the guest agrees, the guest can book for one month and a few days before the end of first month, you can ask the guest to alter the checkout date to the end of second month .... Just make sure that you block all the dates the guest is to stay for.
In terms of payment, Airbnb does not charge the total of three months payment for a three month booking. They also charge on the monthly basis.
2. If a person has stayed for over a month and wanted to extend, you can do the same alteration of the booking. Extend the checkout date. The same long-term rate will be applied for extended days. I have done this a few times to my long-term guests.