After you have got the national dex, go to pal park and talk to Professer Oak. He should in there. Afterwards, he will leave. Now head to Eterna City and approach the house across the street from the Gym. Enter the house and talk to professer Oak again. He will say that the legendary birds are in Sinnoh. If you look at your marking map application, you'll see the roaming pokemon. Chase them and they will battle you. Save the game. Make sure you use mean look as your first move and ,by the way, your pokemon's speed must be faster than theirs. They know roost so after using mean look try a move such as heal block. Once you get their HP to very low, throw an ultra ball (if daytime) or dusk ball (night time). If it faints quit without saving the game so you can try again. Good luck dude!
You say '''''' I'm just kidding you stink if you havent got it
Complete the sinnoh pokedex
If you have a national pokedex then go to eterna city and on the left across from the bike path in the house you will find professor oak. Talk to him and the three legendary birds will appear on your tracking map.
Make sure you go to the pal park first where oak is then go to his house in eterna city. Assuming you have completed national dex and elite four
I have done that now, but they wont appear! Is it because I still haven't caught Cresselia and Mesperit yet?
First you need to get the national pokedex and them go to Erthern city and the house where nobody lived before go into it and pro. Oak should be there and he'll tell you that the birds are in Shinnoh