MGM Junior camp is the most popular.  It's full of talented pro shooters teaching juniors.

TDSA Tulsa had a really good intro pistol class they call Combat Pistol 1.  I took it a tar after I started and it corrected all of my bad habits.  I highly recommend it to anyone.

It was t the first class I took simply because the name had me thinking Timmy Tactical and I was wanting competitve shooting oriented stuff.  Well it should have been the first class I took.  All four of the competitve pistol shooting classes I've taken we're slightly more advanced.  They assumed you had already been taught the proper way to shoot your pistol.  I learned on the Internet.   So to me it looked right but they straightened it out.  

I feel the pistol is by far the hardest of the 3 guns to master and a solid foundation in pistol will translate over to better performance on the long guns.

If you're looking for a 3 gun specific course there aren't many that I'm aware of. Kurt Miller is teaching over at Alpha Training Academy in Pryor, OK (1/2 hour east of Tulsa) later this month.  His 3 gun class was the first class I ever took and it took me far.  Id highly recommend it.