There are several Legendary Pokémon that you can encounter in Pokémon Brilliant Diamond and Brilliant Pearl. You'll be able to find them when you reach the end of the game and unlock the game's national PokéDex, so you don't have to worry about them before this point. In this guide, we will detail how to find Articuno, Moltres, and Zapdos in Pokémon Brilliant Diamond and Brilliant Pearl.
You will be able to find Articuno, Moltres and Zapdos along with the other Legendary Pokémon, and this is at Ramanas Park. Similar to the national PokéDex, this location does not open until you reach the end of the game and can access this content.
When you do, you will have to work through the various lists available at the park. These lists require mysterious fragments, which you can find by digging for them in the Grand Underground through the fossil digging minigame. You can increase your chances of mysterious shards appearing by increasing your Diglett and Dugtrio bonus while in the Grand Underground.
Articuno, Moltres, and Zapdos are the Kanto lists, and it's important to note that Articuno, Moltres, and Zapdos are all exclusive Pokémon Shining Pearl Pokémon. If you own a Pokémon Brilliant Diamond version, you'll need to trade with another player who caught them in Pokémon Shining Pearl.