Drop/leg holsters are cool for most outlaw 3gun formats. I cant speak about the IDPA 3gun stuff that is going on around the cities, its not my scene.
The sporting clays pouch is OK, I know a couple guys who use those on longer shotgun stages but you will be better served with some of these: http://www.midwayusa.com/find?userSearc ... hell+caddy
I only have three of the 4rounders, an arm band that holds five, and a spandex holder on the shotgun stock that holds 4 ($6 at Walmart) plus 9 in the tube of the gun gives me 30rds total. I have shot several local matches and one major. This was enough for everything I encountered with the exception of one stage where I had to borrow another caddy from a fellow shooter.
1) Practice loading the shotgun! Its the biggest time sink in 3gun and most people are weakest at this skill. I would also consider a 8-10rd tube a must have!
2) Know your rifle and pistol zeros! I would say 25% of people show up with no DOPE on the rifle at all and a 50yd (or less) zero. That's cool for 75% of the stages but the other 25% require you to know how the rifle is shooting at longer distances (200yds on the rifle, 25 and 50yd DOPE on the handgun, and 50-100yd slug DOPE on the shottie) is a bare minimum IMHO.
Most local shoots you will be good with 100rds for each gun and 10 slugs. I always bring 300rds for each weapon in case I have re-shoots or someone else has a mechanical and needs to borrow my gun (I would want them to use factory ammo or my ammo only in my guns).
Got questions? Let us know! We are all here to help, feel free to PM me if you need anything. I have loaned guns to several of my buddies to try out 3gun I am a relative new comer myself (in my 2nd year) but I am having a blast and getting a little bit better on each outing!