Gems can be found on enemies that carry loot (white triangle on the hud and white in wraith mode), at destroyed monuments, and completion of challenges in missions.
Once you have the same three gems, head to the inventory, ensure none of the three are currently equipped, press
to Slot Gem. Then hold
over the three to Combine Highlighted Gems and it will unlock.
If you want to improve the quality of gems dropped by enemies, you can purchase an upgrade in the skill, if you have collected all Gondorian Artifacts. First, purchase the Treasure Hunter skill (5th option of the Wraith Skill tree) and then the Upgrade: Prospector (option 3 of Treasure Hunter). Note that this is merely an option and you can easily get this achievement without it, but it may help when trying to forge a top tier gem for Master Forger in Middle-earth: Shadow of War