Guys, sorry for the delay. I was shooting the 3-Gun Nationals at USSA and didn't have access to a computer.
Sling choice really depends on what you want the sling to do.
I'll start with Secesh
what is the best sling to use in 3 gun?
I prefer the old style 2 point sling attached at the front and rear sling swivel (or the 1907 variety.) These type slings are used to carry a rifle and to increase the stability of your shooting platform. Probably the best methods of it's use is still found in the Marine Corps instruction manual. I would highly recommend obtaining a copy of the manual if you are considering using this type of sling. I find it quite useful in 3-Gun competitions.
For CQB applications, I generally prefer a 3 point sling vs. single point slings. 3 points will give better control in extreme CQB applications, but they are more cumbersome and can block access to certain AR15 controls. The single point does not hinder access to certain controls, but also has less support and retention capabilities.
Lately I've been working with the Viking Tactic's 2 point sling and it's growing on me. It may become my new perference in slings, but I need to do some more work with it before I switch over to it on a regular basis.
Erik