If your 3-gun matches are anything like ours, you WILL need some choke on your shotgun. I've seen guys show up with their cylinder bore riot guns not be able to take down steel targets without a couple really good hits or using really stout ammo. I started out running a skeet choke and was deadly on aerial targets, but had to get perfect hits on steel for them to fall reliably. Now I run a light modified choke for just about everything.
My 3-gun shotgun has a 24" barrel, which is about even with the end of the extension tube. I have never felt like it was too long, but I do think it helps me get hits on the aerial targets. I just picked up a pump gun for Heavy division, and specifically looked for a gun with a 24" barrel so it would handle like my current gun. The extension tube will also go out to about the end of the barrel on that gun.
Good luck!
-Randy