Type trios

-Fire/Grass/Water: Conventional starter trio.

-Fire/Ice/Electric: Conventional legendary trio.

-Ground/Water/Flying: Weather Trio

-Ghost/Steel/Water: Creation Trio.

-Dragon/Fairy/Dark: Mortality Trio.

-Flying/Ground/Electric: Kami Trio.

-Fire/Water/Electric: Legendary Beasts, OG Eeveelutions.

-Rock/Steel/Ice: Legendary Golems

-Psychic/Steel/Dragon: Nebula Trio.

-Fairy/Fighting/Poison: Dynamax Trio.

-Dark/Psychic/Fighting: Gen VI Starters

-Fighting/Steel/Ground: Gen IV Starters

-Fighting/Dragon/Ground: Gen III Starters

-Dark/Fairy/Ghost: Gen VII Starters

-Steel/Rock/Grass: Swords of Justice -Keldeo.

Quartets:

-Grass/Water/Psychic/Electric: Guardian Deities of Alola

-Water/Steel/Grass/Rock: Swords of Justice.

-Rock/Steel/Ice/Normal: Legendary Golems + Regigigas.


Remaining Starter types:

-Water/Fighting

-Grass/Dark

-Fire/Ground

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Edit (27.11.2019): Added US/UM and Sw/Sh additions.