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Hyper Diamond Lvl3 is obtainable from the Jewelry Store Robbery, in order to get a lvl3 one, you need to have a lvl2 hyperchrome in your inventory.
Diamond LVL3 hasn't been all that wanted recently, consistently being lower demand than M12, and consistently being the least wanted LVL3 by a long shot. Most regular traders wouldn't trade for this and would rather trade for other items at this price range, making the item quite unwanted, and the demand low.
Supply is widely distributed — no single holder owns enough to register as a hoarder.