When I was experimenting with 3 under, I found that the double nock set was almost a necessity for consistent accuracy. I've tried it shooting split fingers and haven't really noticed a difference one way or the other, so I just tie on one nock now.
If you shoot three under and tend to pull a bit more with the bottom finger, I think it changes the angle of the string below the nock enough to let it slip at times. I also found that the double nock set pinched the nock at full draw slightly and gave me a more secure feeling about the arrow.
The only downside is if you get excited in the heat of the moment and nock the arrow below the bottom nock set. At least that is what I am told. I always have an arrow on the string already when I'm in my stand anyway, and I have only ever gotten one more shot at a jake once out of a ground blind...missed both times, but I can't blame it one the bottom nock set. I was only using one. I was shooting through the screen and my depth perception goes off kilter when I do that.
Anyway, glad to hear it's working for you.