Although it might cause things to be more complicating, I honestly wouldn't mind if they had introduced this form of system. They could make trial-typing exclusive to only some limited amount of legendary, maybe. Legendary Pokémon don't usually get used in competitive play, so it wouldn't really make much of a difference in terms of that. Two Pokémon that could possibly have three types is Black Kyurem and White Kyurem (they are considered separate, aren't they?). Former could be a combination of ice, dragon, and electric, and the latter could be dragon, fire, and ice. d: