It'd be great if you'd give us some ideas to start with, like Pokemon you're interesting in using, your playstyle, etc. Telling us to build a whole team for you is a bit much. There's so many options to consider, and its YOUR team, not ours. Give us a little something to work with here.
Does anyone even battle competitively on Gen 3 anymore?
yes people do lol, not many tho. adv lc was tbh one of the most fun tiers till date, and gen 3 ou is amazing you should try it.
also this looks like help for in game team, take it to ingame section.
anyway this should help you in deciding whats good and whats not, after you have decided what you want we might be able to help you, pls dont select the top 6 mons lol :)
http://www.smogon.com/forums/threads/adv-viability-ranking-ou.3503019/
hope i helped.