Upgrade from 3-Months to a Year of Playstation Plus [duplicate]
Solution 1:
Yes, the PS+ time you buy is cumulative. This means that you basically have a buffer of 3 months time left. When you add 12 months, you'll have 15 months left.
So once your first 3 months expire, you'll still have 12 months in your account, which will be used automatically.