Here is a tip. Many archers will overtime allow there fingers to uncurl from the string prior to the loose and on the draw. This is a bad habit that happens quickly with some and develops over years with others, but it leads to a lose of control.
I think with threeunder there is less of a likelyhood of this happening due to there being no arrow nock between the first and second finger. Next time you shoot, as an experiment, shoot split finger but when you "hook up" on the string, middle finger on the string first then place the other 2. Really curl your fingertips back towards you, I mean past 90 degrees. Hook all the way to the first joint, even slightly past on the middle finger. As you draw back, PUSH your drawing elbow back, anchor with middle finger in corner of mouth, anchor, keep pushing the draw elbow back and REFUSE to hold. Allow the back of the hand to relax, not the fingers. If you have tension in your back, and you will if you drew right, the string will in essance push the fingers out of the way at the same time.
The other issue is string twist. If you will stand in front of a full length mirror and draw a bow, (don't shoot the mirror, the wife will get PO!) you may see the string twisted where your fingers are, doin a weird "S". For a release to be executed properly you need to have you third finger well into the string, your elbow in line with the arrow from side view, and your string with as little twist at full draw from the critter's view (frontal/mirror).
I believe it is easier for 3fingersunder to "hook up" on the string, ergomonically if you will, in the proper position prior to the bow being drawn. I believe that 3under is no more inheirently accurate than split if both are EXECUTED properly.
If you shoot gap, I believe 3under is probably better to cut the distance from eye to back of arrow to point, lowers your point on, and does give a solid anchor. It feels weird with a longbow to me(maybe it is the lower mass bow weight, maybe it is in my head!). With a recurve, 3under is not an issue for me.
I shoot split with LB, try hooking up well curled, refuse to hold instead of releasing, push draw elbow back instead of pulling the bow back. I believe if you give this a try for a while you will notice less left, right and up and down issues with arrow flight. Do not try to hit anything for at least one to two weeks. You are reprogramming your hair covered computer. I would be willing to gues you will cheat and shoot before the week is up uop at a "target". You will probably notice improvement, but your other habit will come back quickly. Give it time, and an honest try.
Shoot at a blank bale and just pick a spot. If you try to hit a target like a bullseye and change your technique it just wont work. If you are shooting at a game animal, concentrate on a small spot only. Do not think about form/technique if you are shooting at something that will bleed. You got what form you have trained at that moment. If you can get your hands on the book "Understanding winning archery" by Al Henderson, he goes into detail about this.
Longbows & Short Shots,
Jeff Schulz
PS: Have fun with archery, always works towards perfecting your craft, and pass it on. It truly is; as Jim Ploen used to say the "Sport of Kings".