Most of the 3 gun matches here don't allow slings... but they get in the way anyway.
They don't use slings in 3GN either. Nor was the 3 gun match I shot in Texas.
I like using the Magpul MS3. I like a one point sling for my AR, but can run it as a 2 point sling if wanted. Really any 2-1 point sling would work for me!