The 3 gun match out at rio is actually a 2 gun. (Evidently there are pistol matches every night, so there isn't really a need.)
http://www.riopractical.com/ Click on the Rifle/Pistol link on the left.
Keep your eyes open for Cav Arms 3 gun match class. They offer a free class on a regular basis that will walk you through the gear you need, the range rules and really get you started. Probably most importantly, you'll also meet a few folks so your first match will have some friendly faces.
The folks out at the Rio two gun match are extremly welcoming. If you go in with an attitude of wanting to learn and play safe, you'll find folks will bend over backwards to help you get started. It's a great crew and a whole crap load of fun. (Cav Arms has a gallery of pictures with some from the Rio matches.)
Also, don't get hung up on the gear. I've seen guys go out and shoot with an SKS and a hunting shotgun and have fun doing it. Heck, I did my first two shotgun stages using a side pocket to hold my reloads. :P