Flores to my understanding the 12+3 month offer was only for a limited time through the playstation store. I do not believe that offer is currently available (I can't check since the store has been down nearly a month now). However if it is available again I would personally purchase the subscription through the STORE then.
The 12+3 deal might be a store special, where as these Playstation plus cards are designed to give a static face value of 3 or 12 months. the 12+3 offer was a special offer through Sony's Playstation Network Store. These Plus cards are reletively new (barely 2 months old).... Even if they did give 12+3 I personally wouldn't do it anyhow for one reason
* The card says "12 month subscription" *
Legally the offer is for 12 months so if there is a mix-up it would be very hard to prove you were entitled to a 12+3 deal... where as when purchased through the Playstation Store (when the sale was going on) the item was listed as "Playsation Plus 12 month subscription w/ 3 months additional service" I believe. So your receipt and item description has the 12+3 in it.