What trio is collectively known as the legendary birds in Generation I Pokémon games?

Articuno, Zapdos, and Moltres are the legendary birds of Generation I Pokémon games.

The three legendary Pokémon represent ice, electricity, and fire respectively. In the original games, players can encounter them as rare, powerful Pokémon outside the normal starter and Gym-battle progression. Their locations are associated with major areas of Kanto: Articuno with the Seafoam Islands, Zapdos with the Power Plant, and Moltres with Victory Road in the original versions.

The trio’s names share an avian theme, while their names also incorporate Spanish-derived numbers: uno, dos, and tres. That naming pattern is a useful clue, but the group’s official identities are the three species themselves rather than a generic phrase such as “the elemental birds.”

Later games introduced other legendary trios, including the Johto trio and the Sinnoh lake trio. Those groups are separate, so their members are plausible but incorrect answers to this Generation I question.