You can't get them... you have to get them from the Hoenn region games like Emerald, and Migrate to ss/hg or platinum. Sorry but you have to get a Hoenn game.
Hope this helps!! :D
The 3 Regi's can be found in Platinum but they cannot be legitimately obtained without cheats. An event a long while back distributed access to catch Regigigas in the Snowpoint Temple. After you catch it, the other Regi's will be available to be obtained in their caves at different locations. Regirock is in its cave on Route 228 towards the entrance to Stark Mountain. Regice is in its cave in Mt. Coronet near the entrance to Route 216. Registeel is in its cave on Iron Island near the exit. All the Regi's but Regigigas are Level 30. Regigigas is at Level 1.