If you paid money to keep Pokémon Bank after the trial period, then all three can be yours starting on February 27! This would've made for a nice birthday present had I actually kept Bank, but oh well. I'll just look for some on the GTS like many did for Serperior.
All these recent developments really cracked down on the list of missing hidden abilities. The gen 1, 2 and 3 legendary trios, Heatran, Aurorus and the small and large Gourgeist varieties are the only ones left.