How likely is it that a 1-3 mm furcation in a back tooth can be maintained by periodontal cleanings every 3 months in most people? pocket depth = 5 mm

No way to predict: It depends upon many factors- namely your age and general health, how well you keep the area clean, how well your dentist is able to clean the area, your recuperative powers, etc. Some luck can also be a factor as well as general stress levels. I have seen patients maintain such situations for a decade or more, while others have flare-ups and failures within a year. Consider long term prognosis.

Somewhat likely: With great home are and your periodontist on board, it could be stable for quite a while. Have your periodontist make sure the bite is correct and the molar is contoured properly to avoid food impact ions into the furca!

Furcations: Given the proper tools for oral maintenance along with adequate access and attentiveness , a patient should be able to maintain molars diagnosed with a type i to iii furcation involvement in conjunction with appropriately advised professional periodontal cleanings. There are various factors specific to that individual which would need to be identified for a more definitive prognosis.

Most cant: Patients have difficult times cleaning out furcations correctly. Eventually it leads to further breakdown. Better to address problems early and possibly avoid costlier procedures down the road. Maybe even having to remove the tooth if it reaches a level 3.