Ok, I run a Tactical Tailor 3-point on my patrol carbine, and I have a CQB Solutions (Now Spectre) on my Dissy. A 3-pt can be used as a 1-pt by simply moving the slider that is on the portion of the webbing closest to the gun rearwards towards the rear sling hookup. What this does is takes the loop and pretty much frees everything up and makes the loop around your body conform to your body instead of the gun. It will also make the gun hang straight down and give you the ability to shoot from either shoulder like a 1-pt.
A 3-pt can also be used as a 2-pt, simply by legnthening the strap that goes around the body, and then just wearing it with no webbing between you and the rifle-all of it goes over your shoulder.
To me, if you don't mind making an adjustment or two when yu need a particular method of carrying,t he 3-pt is the most flexible platform you have, as it does it all.