Is Jirachi, mew, and celebi considered a 'trio'?

Question

Like the genie trio landorus, thundurus, and tornadus

Answer 1

No, because they're from different Generations. Mew is Gen I, Jirachi is Gen III and Celebi is Gen II.

Trios are Pokemon like Kyurem, Reshiram and Zekrom which are from the same Generation and the same region. Usually they will be referred to as a trio in the anime.

List of Legendary Trios

Comments

This is not true, as Regigigas was added to the regi trio, or rather, regi quad. Though it is a quad, it still confirms that Gamefreak does infact add Pokemon into duos/trios/quads/etc after the groups' original release.

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yea, but Regigigas is directly linked to the other 3 Regis. They're needed to unlock Regigigas.
Jirachi/ Celebi/ Mew have no link with each other, apart from all being psychic type and small.