Not sure why it's 3 years specifically, but I'm sure they have a limit in case they change prices in the future (be it charge more, make it free etc).
If they charge more, they don't want people paid to far ahead as that's lost revenue they could have gotten on the person paying the higher fee on those additional years. If they make it free, they'll have to reimburse people for outstanding years paid (give them MS points most likely) and by having a limit they limit the amount of years they'd owe people if they go to a free service.