How come every generation's legendary pokemon has a trio?

Question

In kanto they have
moletres,zapdos,and articuno

in johto they have
raikou,entei,andsuicune

in hoenn they had (yes now it is four but originally)
regice,regirock,and registeel

in sinnoh they have
uxie,azelf,and mesprit

i forget about unova's but I know they are
flying,electric flying,and ground flying

Answer 1

They all have a tie in with the mascot legends. The lake Trio form the chain that holds down the mascot legend, the johto Beast trio help protect johto and were revived by Ho-Oh. the Regis were created during the stone, ice and iron ages of human technology and awaken Regigigas who I think formed the earth. Go to Bulbapedia for the whole legend

Comments

the unovas are tornadous, thundourous, and landourous

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As well as Terrakion, Virizion, and Cobalion.