Quoted By:
>gen 1-2 main legendaries: No third legend
>gen 3-4-5-6: Third legend is a key member of the trio that brings balance to the dual legendaries
>gen 7-8-9: Third legend is a completely separate existence which only happens to form a trio with the dual legendaries by chance.
Why? Necrozma's relationship with Nebby's line being "he's the big boy and Nebby is just a good snack for him" was odd already, but then Eternatus being just a very big monster that a pair of wolves defeat everytime it wakes up was dumb, and now the hidden third legendary is the only "real" legendary and Koraidon and Miraidon are just overpowered paradox mons (aka: imaginary prehistoric/futuristic versions of normal pokemon) that the third legend created.