Ok, to get the 3 legendary birds, (moltres, zapdos, and articuno) u have to first beat the elite four and get the national dex. after that, u have to go to eterna city and go into the house on the nearest left of the entrance to the bicycle route. (if he is not there, surf from the route below sandgem town and get to pal park first) after u enter the house, talk to him and he will mention something about the 3 legendary birds being sighted in sinnoh. then, use ur marking map on ur poketch and they will show up as markers. just make sure it's not mesprit or cresselia, since the markers all look the same. (if u don't have the poketch app marking map, go to jubilife city and talk to the president of the poketch company until u get one).