It really depends on your use of the weapon.
Single Point slings are particularly suited for people who intend to use the weapon and need it "at the ready". The very nature of the single point is that it allows the user great freedom in maneuvering and manipulating the weapon, while still giving the user weapon retention. Allows the user to shoot from either shoulder with the sling on (like when shooting from cover) and good for PSD, getting in and out of vehicles, dynamic entry, MOUT, etc...
The downside is that the single point will dangle from the user if he doesn't keep both hands on it, like if you transition to a sidearm, need to climb something, slapping cuffs on someone or if you are carrying wounded. You can still transition to a sidearm and all, but you will need to move the weapon down and away from your front and if you are running or climbing, etc... , the rifle can bang against the user. If you go to your knees without your hands on the weapon, the front end can smack the ground.
Some guys even complain that if they simply let go of the rifle it will hit them in the nuts... which of course is not a recommended way to handle a weapon with a single point sling.
2-Points are good all-purpose slings. Mostly just a carry strap. But it is a more secure method of keeping the weapon on you.
The problem with the standard 2-point is that once you set the length, it is stuck at that length and usually takes a minute or two to take off the sling and re-adjust the length.
With a 2-Point Quick Adjust, you can adjust the length of the sling in a second while you have the sling on, and can still carry it "at the ready". It combines the best attributes of a single point and the best attributes of a 2-point. A 2-Point Quick Adjust is really the best all-purpose sling.
The 3-Point has only 1 real advantage, and that is that when you transition to a sidearm, the weapon is held at a more natural position for you to do so.
It is a bit complicated though for many and makes the weapon almost too secure. So much so that shooting from a weak shoulder (Around cover, or if you get injured on your strong side) is impossible with the sling on.