While having all three at level 100 in the main series Pokémon games doesn't give you any specific in-game reward, there are a few things to consider:
* They're strong: Having all three at level 100 gives you a formidable team for battling.
* Completing the Pokédex: Catching and training all three Legendary Birds contributes to completing your Pokédex, which is a goal for many players.
* Trading and Battling: Having all three at level 100 makes them great candidates for trading with other players or for competitive battling.
Important Note: The specific effects of having all three legendary birds at level 100 can vary depending on the specific Pokémon game you're playing.
For a more specific answer, please let me know which Pokémon game you are interested in.