100% positive that subscriptions stack, so if you add the 12 months you will have 15. However, if you are auto renewing with your credit/debit card every 3 months I can not be certain Micro$oft won't stop charing your card. The best thing you can do is ring M$ and get them to remove your card from the system because you can't do it yourself from your xbox. Some of the agents can be arseholes about removing your card SO BE PERSISTANT! They refused to remove my card several times and I was very close to losing my temper.
I don't know why they refuse when they know full well they can't force you to pay by card, but I guess it's Micro$oft, so I do know why huh.