I would like to learn more joseki about playing on the 3rd line. Things like when you have two stone on the third line with a 2 stone jump.
I can't do better than just dumping links on you, so here goes:
Extension, TwoSpaceExtensionOnTheThirdLineCannotBeCut, TwoSpaceExtensionUnobviousAnswers,
As far as responses to the play on the third line, there is no general theory, and it all depends on the position. But you might look at Invasion, Cap, or do a Position Search on something you're interested in. Also see the BQM series. --Hyperpape
Attaching to an invasion is often a bad idea. Attaching tends to make both stones involved stronger, and when your opponent invades, you will often want to keep his stones weak as long as possible, because you start out at an advantage (he has invaded your position).