Nathan,
Generally, a 3-point sling is used to allow the rifle to hang, but remain accessible, without flopping around. Often, they're also made so they can be set up to allow the rifle to be brought into firing position without having to adjust or manipulate the sling in order to do so.
"High speed, low drag" type of thing...
I like the idea, and borrowed two from family members at different times. But never could sell myself on the idea - even for short, light rifles; but especially for 'normal' rifles (40"+ long and 8-10 lb).
Where I hunt, the terrain dictates that the rifle(s) be hand-carried very frequently - whether for safety, to keep the muzzle (or butt) from bashing on rocks, to keep the muzzle clear of snow on the ground (or water on branches), or to maintain balance over rough areas and keep the rifle ready to be protected in the case of a fall.
So, having a sling that's easier to remove from my body is better than a rifle that's always 'at the ready'.
The closest I get to the 3-point idea, is when I pull the shoulder strap off of my range bag and clip it to my AR pistol as a 'single-point' sling.