Niantic has announced the official start and end dates for the original legendary bird trio in Pokémon GO via twitter!
Articuno: now until July 31st
Moltres: July 31st to August 7th
Zapdos: August 7th to August 14th
Like Articuno and Lugia, Moltres and Zapdos will only be encountered and caught via Legendary Raid Battles. Each only last one week, so catch ’em while you can! No official end date was given for Lugia.