You need 9 adamant ingots to forge all 3 of the Legendary weapons (3 ingots for each).
According to the "Fire Amid the Water" quest text, ingots can be found in archives and repositories in the Oikos of Atlantis region, or alongside Keeper's Insights in the Porimos Ring region. They are in chests. To make them easier to find, stay outside and scout the area inside the entrance with Ikaros to mark enemies and special items.Locations of All 9 Adamant Ingots
The Forge of Atlantis is where you need to make your weapons. It is located at the gold ? symbol on the southern part of the central section, Royal Mountain of Poseidon. The ? will be replaced by a special icon fast travel point after you discover it.
When you have 3 or more ingots you can forge a weapon at the Forge by interacting
with the molds in the forge's upper section. Choose one of the 3 classes (Warrior, Hunter, Assassin) to begin the process.
Your reward's properties will be based on your choices. You can only forge 3 of the 9 possible weapon combinations, so choose what best suits your play style.
The achievement should pop as soon as you start forging your 3rd weapon. If not, Kennyannydenny
says, make sure you have all 3 weapons in your inventory and not in storage.
All Isu Forged Weapon/Class Stats