First, I learned to shoot in the early 50's, and I can honestly say that I neither saw, nor heard of anyone using "three under" at that time. Of course, our area, and club, were very small. (Wish I hadn't given up shooting for 50 yrs before getting back into it again.
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So, I shoot split finger because that's how I got started and it feels natural to me. Now that I'm surrounded by 'three under' shooters, and can see the advantage it offers in some areas, I've given it a try. It feels fine on some of my bows, but unbalanced on others. I think it has to do with the bow length, or the built in tiller?
The advantage or disadvantage, depending upon how you look at it, is that given the same anchor point, the "spot" on the target will remain visible above your arrow tip/arrow rest for a longer distance for the split finger shooter than it would be for the "three under" shooter.
For the trad shooter who wants to compete in field and hunter rounds, where the required shooting distances go out to 80 yards, being able to see that spot as far out as possible is a distinct advantage, over having the spot blocked in the 40 yd range by the hand or bow, as it is for the "three under" shooter.
For the trad shooter primarily interested in hunting, and/or competing in 3-D where the max distances being shot are 30 or 50 yards, respectively, the "three under" shooter has a distinct advantage, as the 'point on' range for many is in the 40 yard range.
Without getting into the "instinctive vs "gap" shooting debate, the visible gap between the intended impact point and the arrow tip is the width of the index finger less for the "three under" shooter, which decreases the potential elevation error with a misjudgement in the arc of the arrow required for a good hit.
It's a whole lot easier to see than to explain to one unfamiliar with the two techniques. I'd recommend giving them both a try and see what happens. Probably the safest way to try them out would be at 5 yards, or so, using a spot in the center of the butt, and seeing where the arrows impact using the two different finger/nock positions and the same anchor point with your current method of determining where to hold/aim. Just remember that any vertical difference in the impact points is going to be multiplied several times over as the range is increased.
Have Fun!