What if Team Rocket had used all 3 stones on Eevee?

Remember the episode where Team Rocket was about to touch
Eevee with Fire Stone, Water Stone and Thunder Stone at once?
What do you think would have happened if they had? Would
Eevee's body have accepted just one of the stones and evolved
that way? Would it have evolved into something freaky or unstable,
like the fusion of 3 legendary birds in the manga? Would it
have not evolved at all?

They'd obviously evolved even if they didn't want to.
One single touch is enough to make them evolve.

Technically it's nearly impossible for all three stones to touch Eevee at precisely the same time, but if it could happen I'd guess Eevee would take the evolution of whichever stone had a bigger contact surface.

If I remember correctly, in the Adventure manga Red saved an Evee that was experimented by team rocket. After said experiments (In which they used the different stones), that Evee gained the ability to change its form temporarily into the first 3 original Eveelutions at will.

Tho in the end it did permanently evolve into a Espeon...

Correct me if I'm wrong but can't Pokemon in the anime resist evolution? Bulbasaur repressed his evolution in Bulbasaur's Mysterious Garden. Or does that not work with stones?

I think resisting evolution doesn't work with stones. That's why Pikachu slapped Ash's hand away when he was trying to evolve Pikachu before the Vermillion Gym.