@Janet533 You may need to consider following before you choose what kind decision you make
(1) Do you have a monthly discount in your listing? If yes, three months and one month discount are the same. You may just let the guests to alter the period because you still have one month time for your house to be booked by other guests.
(2) Did you give more discount than discount provided in the listing when the guests booked for three month? If yes, you can remove that additional discount and just ask the guest to change the request to normal one month booking.
(3) Do you listings have higher booking rate? If yes, you don't need to worry if your house can be booked after the guest shorten their stay to one month.
(4) As mentioned by @Allison2 , there is a long-term cancellation policy, you may just get one month non-refundable for a strict cancellation policy. And the guests are still staying for one month anyway, you may consider to accept the guests alteration request.
From my experience, I always accept guests' changes of their staying period for longer term guests.