The third profession comes from the Saga quest of your class: you take it from level 76 at your class master, and it changes your class itself at the end. Every Saga follows the same path: six Tablets of Vision with Resonance Amulets, fights with monsters at the tablets and the Archon of Halisha. Only the NPCs, monsters and items differ.
A Saga needs items from other quests: the Ice Crystal from The Finest Ingredients - Part 1, the Divine Stone of Wisdom from Magical Power of Water - Part 1 (alliance with the Ketra Orcs) or Magical Power of Fire - Part 1 (alliance with the Varka Silenos), and for the Saga of the Duelist also Top Quality Meat from Delicious Top Choice Meat. They are in this list too.