There is no way to do in Pearl, however you can get them in Pokemon Platinum.
Regirock is found, hidden in Route 228. (Rock Peak Ruins)
Regice is found, hidden in Mt. Coronet. (Iceberg Ruins)
Registeel is found, hidden in Iron Island. (Iron Ruins)
Remember, those locations are only accessible in Pokemon Platinum. You can trade back those Pokemon from that game into Pearl, keep them all in your party and head to Snowpoint Temple to encounter Regigigas.
Hope I helped. :)
Sources: Experience, and each of the Regis Serebii pages.