I switched to three under when I realized that my inexpensive bow stacks bad and pinches worse. My arrows were all over the place with a one-over grip, and though it was awkward at first, I immediately saw my groups improve.
For me, it was getting out of the way of the arrow, and then also sighting down the arrow more easily than with a one-over grip. Of course, that means I can't shoot as far, which is way better for me than false confidence.