Hi there, once on the listing under availability you should be able to book it for the time you require as long as the listing is available during those dates, if it's not you can consider another listing or sending enquiry to the host to check on availability, hope this helps!
@Milton49 Are you a host wanting to put your settings to take reservations 3 months into the future? (Your post was rather lacking in detail, and another responder assumed you were a guest asking about booking)
If so, go to your calendar page, click on "Availability " at the top of the calendar, and you can there choose your booking window.