"Vapreon: no matter what Pokemon Vaporeon is paired with, it helps out its partner. It can learn TM28 Dig (that helps with Electric-type weakness), Ice Beam, and Blizzard (both help with Grass-type weaknesses). Vaporeon has the best moveset variety out of the three.
Jolteon: Jolteon can learn Tm18 Rain Dance and Tm25 Thunder in combination. Using this combo (Rain Dance first, and then Thunder), Jolteon can deal awesome damage to and Pokemon vulnerable to Electric-type attacks. Its stats are strong, its best being HP and Sp Attack with a Beneficial nature.
Flareon: Flareon has probably the worst moveset pool out of the three. Knowning only 2 HMs (Strength and Rock Smash), thats a pretty bad disadvantage. That doesn't really help any teammates except Dark-types and Normal-types."
some helpful info for your decision