Help adding Target gift cards

We received a couple gift cards from my in laws and I can't add them to my target account for some reason. It will accept the card number on the front but then asks for a pin. I don't have a pin for these 2 cards other than the 3 digit code on the back and they don't work. What do I type into that box? I'd really like to use these online as I can't really travel the almost 2 hours to our nearest Target.

*Update* The cards look like the one pictured below.