I've been using a 3-point sling of one brand or another for ~10 years on various guns. On a few guns, what ended up always working for me was a simple 2-point sling or using the 3-point as a 2-point.
3-point Slings:
-easy to transition to handgun by just dropping long gun, although you may have to sweep the long gun to the side.
-there's a method of swinging the gun over the shoulder to carry, but he gun always wants to go horizontal behind your back, unlike just a 2-point sling, which will stay diagonally across one's back.
-with a shotgun, if a sidesaddle is used, there's a strap which gets in the way of accessing the sidesaddle, although just practicing with the sling allows you to be aware of this to work around this problem. I also didn't like this sling using an M1A, but it worked OK using an AR-15.
-a little different practice in technique vs. a 2-point, but I wouldn't say more practice, just different.
See how one of the straps is all but over the sidesaddle.
2-point Slings:
-similar techniques for various long guns used, including muzzle-up or muzzle-down carry techniques.
-simpler, less "strappage" in which to deal, less tangling up.
-transitioning is quick, but does involve putting sling over arm, shoulder, &/or head.
I haven't used the latest Viking Tactical 2-point slings, but others swear by them, but again, a little too much "strappage" for me. I also haven't used any single-point slings.
There is one sling which behaves mostly as a 2-point, but is quickly converted to a 3-point by "Uncle Mike's" which I also like:
http://www.unclemikesle.com/products/slings_swivels/tactical/slings_shotgun_sling.html