I will now proceed to list the cuck/third wheel of every trio. These mons are the most forgettable and forgotten by the fans, at least compared to the other two thirds of the trio.

>Legendary Birds
Moltres
>Legendary Beasts
Entei
>Weather Trio
Kyogre
>Legendary Titians
Regirock
>Creation Trio
Palkia
>Lake Guardians
Uxie
>Tao Trio
Kyurem
>Forces of Nature
Thunderus
>Swords of Justice (+Keldeo)
Terrakion
>Aura Trio
Zygarde pre gen 7, now probably Yveltal
>Light Trio
Necrozma
>Dogs and a Giant Hand Trio
Zamazenta