On Pokemon Showdown! It lists OU tiers from past generations, but there is no Gen 3 OU tier.
Probably because no one really plays it anymore, and besides, if you do, you can always go to Custom Game. This lets you use anything, so make a Gen 3 team and you can play there :) Also I'm pretty sure gen 5 / 6 OU lets you use stuff from past generations.
But the main reason is that barely anyone does anything related to Gen 3 nowadays.