The three weapons are the Stormslinger, the Icerail, and the Forgefire. The Stormslinger is given to you by Ourea early on in the DLC, when you first meet her. The Icerail is given to you when you complete the hunt against Aratak. The Forgefire is obtained by clearing out the bandit camp located in the DLC area of the map.
Once you reach Longnotch, you can talk to Varga to receive three errands centered around upgrading these weapons. Once those errands have been completed and weapons upgraded, you will receive the trophy.