The Y3 Thorn is a different weapon from the Thorn that unlocks the Grimoire card. The same is true for the Icebreaker and the Gjallarhorn from Y3. If you want to unlock the Thorn Grimoire, you'll need to get it from a Legacy Primary Exotic Engram sold by Xur. Icebreaker and Gjallarhorn can be gotten from the exotic chests from the Y1 version of the Vault of Glass or Crota's End raid OR from a Secondary/Heavy Legacy Exotic Engram sold by Xur. Thorn was a step-based exotic, you had to do bounties and specific steps for it, so it wasn't added to the original Y1 exotic chests' drop pools, so the only way to get it at this point is through the Legacy Engrams sold by Xur. Also, the forum is Bungie's customer support area. Most questions can be answered by regular players because the information is out there and easily available, people just don't actually go and look for it so immediately jump on Bungie to ask a quesiton that's already been answered a few hundred times. In the instance that a question is asked that cannot be answered by the community, then a Bungie rep will actually reply. Your question is one such question that has already been answered a multitude of times and is on Bungie's current issue list.