Instructions
Insert your copy of "Pokemon Pearl" in the top slot, and insert your GBA game into the bottom slot. Load "Pearl" to the main menu as normal. Tap "Migrate," an option on the main menu. This takes you to a new menu.
Tap "Articuno," "Zapdos" and "Moltres" to migrate over. You must also pick three additional Pokemon, but the ones you pick do not matter. Load your saved game in "Pearl" after the Pokemon migrate.
Travel to the PAL Park, located in the southwestern corner of Sinnoh. If you haven't been here before, you must play the game until the game takes you there naturally. You cannot enter Sinnoh prior to this.
Speak with the attendant inside the PAL building. He will give you six Poke balls and allow you to enter the park.
Walk around the tall grass until you find one of the six Pokemon at random. Throw the Poke ball at the bird to capture it. Repeat this until you have captured all three Pokemon.