Personally I wouldn't even count Groudon/Kyogre/Rayquaza and Dialga/Palkia/Giratina as Legendary Trios because although they most definitely are, they're the game mascots and integral parts to the plot of the games, which sort of gives them a category all their own
Dito.
Id also exclude Reshiram/Zekrom/Kyurem from the list, and perhaps even Thundurus/Tornadus/Landorus.
Because the term "Legendary trio" really foremost relates to the standard first three legends introduced every generation, that are equally special, have exactly 580TBS, and all appear in the middle of the plot before the E4. (The first are mascots too, not to mention Kyurem is quite a different role and concept, and the second are more like Lati@s, the usual extra legends, except they happen to have a third, different superior cousin..)
Which the legendary birds set up.
And has since been continued as a tradition with the beasts, golems, pixies and now the musketeers.
Even though people will argue that there is Keldeo, it is not an equal member, but rather a fourth special member to the trio, similar to Regigigas.
So groups like Kyogre/Groudon/Rayquaza, to me are a duo with their guardian, rather than a trio.