You will find Three-Eyes in the lower courtyard of Skyhold, between the stables and the kitchen. Speak with him to begin the quest ‘Way of the Artificer’. To complete the quest and unlock the Way of the Artificer skill tree, you will need to craft Custom Precision Tools. This can be done at a requisition table once you have the following items:
Codex Entry: Writing on Artificer methods - Found on the table near Varric in Skyhold, who is located in the throne room of the keep. Can also be purchased from the merchant Willvan in Val Royeaux
3x Alpha Quillback Spines - Obtained in the Western Approach region, particularly throughout Giant’s Staircase, Nazaire’s Pass, The Canyons, and Old Prison Road
10x Obsidian - Found throughout the Hinterlands, Exhalted Plains, and Crestwood. Can also be obtained from war table mission Crestwood Resources by selecting Josephine or Cullen
Note: You will need to complete the war table mission Specializations for the Inquisitor before you can find and speak with the trainer.