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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 shown any signs of stabilizing yet, as it's still heavily avoided and lowballed in trading, often going for big underpays of around ~11.5M (sometimes even less). While it's not doing as terribly as before, it still struggles to get base offers, so another value drop should take place.
Supply is widely distributed — no single holder owns enough to register as a hoarder.