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Gen 3 was the only gen of Pokemon that I played competitively, but I ended up becoming quite skilled at it, having an impressive win/loss record on NetBattle as I recall.
It was really Jolteon that carried me to success, Jolteon was the cog that allowed the rest of the team to function.
Scouting with baton pass, healing spike damage taken while scouting via volt absorb, passing subs to Salamence during earthquakes (or just killing the ground type with HP ice on the switch if Salamence was down), revenge killing, easily cleaning up weakened teams thanks to its speed and the lack of choice scarves, shutting down adamant dragon dancers with thunderbolt (for Gyarados) or HP Ice (for Salamence/Dragonite)
Honestly Jolteon never failed to contribute something useful to my matches. Great speed, great special attack, and great support with sub+baton pass.