The three legendary birds of Kanto are amongst the most famous of all the Legendaries in Pokemon X and Y, probably because of how prominently they featured in Pokemon: The Movie 2000.
Either way, they’re pretty freaking awesome, and are always fun to have in your squad.
Unfortunately, you can only capture one of the three in your game, and the choice of which one is decided as early as when you pick your starter Pokemon. If you picked Froakie, you’ll be able to capture Moltres.
If you started with Chespin, Articuno will be your bird, and if you chose Fennekin then you’ll be able to snag Zapdos.
The way this is done is that after defeating the Elite Four you encounter your bird. This puts its entry into the Pokedex and enables you to track it using the location feature on the device.
The location given will only tell you of the route it is on. Once there, you need to wander around in the bushes hoping for a random encounter.
If you do manage to find it, prepare to be disappointed, because it’ll immediately run/fly away.
You’ll have to track it down again. This cycle will continue for 10 turns, before the flighty bird settles in the Sea Spirit’s Den where you can capture it in peace.
Each of the birds have different type weaknesses and strengths, Articuno is super weak to Rock attacks, and also to Steel, Fire and Electric. Zapdos is Weak to Rock and Ice attacks and Moltres is super weak to Rock, while also being weak to Water and Electric.
All three are resistant to Bug and Grass type attacks. Remember that the other two legendary birds cannot be obtained except through trading.