u can migrate them from pokemon ruby or sapphire

It is true you can migrate them after you beat the game and get national pokedex, but
there is also an alternative. If you find a way to get a TRU (Toys R Us) Regigas(event),
you can find three caverns scattered throughout the region. Registeel appears at Iron
Island, Regice at Mt. Coronet Summit(i think), and Regirock at route 228. All of them are
at level 30. Unfortunately the event has passed, but it is possible to find someone
willing to trade it over wi-fi.