It's an interesting question, however... one of the stones is going to reach Eevee first resulting in evolution before the other stones can influence it. Unless you somehow find a way to make the electrons (smallest part of an atom) of all three stones touch Eevee at the EXACT same time. If it did happen though, well fire and water makes steam. Now what does steam and electricity make? Not sure, but I assume it would be a strange energy that could be associated with psychic types and thus Espeon would be the result. Unless somebody here knows what happens when you combine steam and electricity?