depending on the limbs tillering is the real issue here...
the distance that the limbs measure at the point the riser ends and the limbs start(like where the wedge taper ends is a good spot to measure)
most modern custom bows are set so the upper limb is 1/16" -1/8" farther from the string...this accomadates the fact that arrows are shot above the center point of your bow. Especially with an elevated rest...
for me shooting "apache" style I like to shoot off the shelf with the shelf cut as close to the grip of my hand as possible.
the throat of the bow is supposed to be dead middle of the bows length , where your tab and fingers are fulcrumed at release of the arrow...3 under can have the arrow nock at a bit closer to square if the limbs are tillered pretty close...
The biggest mistake made by archers shooting three under is to let their top finger and tab pinch the arrow shaft...this can cause serious "porpoising" ...or even a group that is wide...this comes from the nock not leaving the string with lack of tension on it...paper testing can be all over the place until this factor isdealt with...
if this can be avoided accuracy will double(if not more)...if it is hard to regulate an extra nock point below the arrows nock to allow you to pull off the nock is OK...I like to spin on a little thread under my nock single point to help the nock "lock in place"...also focusing presure equally so that the focus point of the tab at full draw is 90 degrees to the center of the throat also makes a huge difference for accuracy...
I usually focus a bit more on my lowest of three fingers a bit to help balance out the pressure... this has halped me a lot...
I started shooting 3 under about 25 years ago...and anchoring with the nock closer to my eyes sight plane...not really looking at the arrow, but being alighned with it...also canting the bow enough so I can see my target w/ both eyes open is crucial...this also helps arrow flight with bows that are cut to center with the paradox of the arrows flight...
longer bows like a LB seem to be the easiest to shoot this method...but with some fine tuning even a shorter Recurve can perform awesome...
I shoot a 55" TD now...and just like all shorter bows any mistakes you make are magnified...
hope this might be of a little help...
I like to make an "oversized" tab myself to get the best results(with extra material at the lower end...this equals out after drawing back the bow all the way)
it really helps if you can draw out the mechanics going on here in your head, or even on a piece of paper...and see the angles of the limbs, location of the arrow & shelf and nock...also the amount the arrow is above center and the throat of the bow in comparison...it makes even more sense as to why thing work better...
...I have big hands, and the standard tabs just dont allow me to get the even pressure that is so important for consistent shooting...especially with heads...