From Bulbapedia, the community-driven Pokémon encyclopedia.
A legendary trio is a term used to describe a group of three closely-related legendary Pokémon. Legendary trios can be subdivided into two tiers, normal trios and mascot trios. Normal trios, which are always the first legendaries that appear in any Pokédex order, are allowed into the Battle Tower and other battle facilities. They are also the only Pokémon whose base stats total to 580. The mascot trios, whose name comes from the fact that all of them are version mascots, are not allowed in the Battle Tower, and are found close to the end of the Pokédex in the generation of their release. Many trios, in addition to sharing at least one type, will far more commonly share their ability. These trios sometimes have a member seemingly stronger than the other two, like Rayquaza is to Groudon and Kyogre, although this might be like this due to that they don't have a trio master. A trio master is a Pokémon in charge of a trio, in which the trio takes every command of their master seriously or importantly. There are also several legendary duos.
List of legendary trios
