Gen I: Dragonair, Articuno and Gengar
Gen II: Umbreon, Togetic and Ampharos
Gen III: Altaria, Delcatty and Banette
Gen IV: Cherrim, Honchkrow and Rotom
Gen V: Whimsicott, Frillish and Cinccino
Gen VI: Aurorus, Meowstic and Slurpuff
Of all, I would say Altaria and Banette take the shared cake.
(The stats and how good they are in fights are not counted, even though my teams mainly consist of these Pokémon most of the time)