The problem with 6 o'clock hold is that point of impact, by intent, is not point of aim. So if you set your sights to hit the center of a 6" bullseye at 25 yards, if you switch to a 4" or 8" bullseye your sights will be off. Switch to a 2" bullseye and if you aim perfectly, your point of impact will be 1 inch above the bullseye.
6 o'clock hold is what I was taught, and what generations of shooters were taught. But we were taught when most recreational shooting was target shooting, using targets of constant size at known distances. Hunters didn't zero their deer rifles to aim 3 inches below the intended point of impact -- scoped or open sights, hunting rifles have always been zeroed for POA=POI.
What changed for handguns is the increased freedom to carry for self-defense, and the rise of "practical" shooting competition. We may not be shooting at a target with a constant-size bullseye at exactly 25 yards, so the traditional 6 o'clock hold doesn't work as well. So more and more people are switching (or have never known anything except) center hold.
In fact, from what I've been told by manufacturers, both picture 2 (the one in the center) and picture 3 (on the right) are the same. Supposedly, white dot sights are set up so that aligning the tops of the front and rear sights and placing on the intended point of impact is the same as aligning the three dots and covering the intended point of impact with the front dot.