How do you get Articuno Zapdos and moltres in platinum?

First off, talk to Prof. Oak in Eterna City (he lives in the house in the southwest end next to the gym) and he will the three have been seen around Sinnoh. Track them with your Marking Map Poketch app. If Oak is not in his home, you may have to talk to him first in Pal Park, afterwards he will be in Eterna City.