Like several others, I advocate going directly to the goal itself: Chopin Etude Op 25 / 6.
Since this is apparently the reason for most pianist's interest in playing thirds, why not harness the musical inspiration contained in this piece to build technique? At least for me, I find that musical context and musical results are the most powerful factor in technique development, in addition to knowledge of the workings of the playing mechanism, of course. For this reason, the Chopin Etudes serve both as invaluable material for technical growth as well as a standard for measuring achievement.
The problem of thirds is no different from the rest. There is no special technique for it; you simply (or not so simply, it can be very detailed) apply everything you know about technique, and within the musical context, the execution will surprisingly come together as part of the musical expression. It really is a source of wonder to this 66 year old, whose fingers feel very clumsy tapping double trills on a table top, yet at the piano playing this etude just today, there was a marked leap in ability. I will say that there are some physical issues occuring with my arm that may be related to it. If I learn more about it, I will post it. It could be pretty interesting.
Richy