If we were to evolve Eevee with three evolution stones, I'd imagine it to evolve with three different typings. Unfortunately, latter doesn't exist, so it might not be an actual thing any time soon. Maybe they could still allow us to evolve the Pokémon this way, but have it gain only two types instead of three (depending on how you order the stones when using and whatnot)?
Nonetheless, this would be interesting, and I would very much like to see it, personally. More Eeveelutions there are, the better! They don't have to do this stuff with Eevee necessarily; it could be an entirely new Pokémon all together. I'm sure much people wouldn't mind, and neither would I.