Red:
In simple terms, you want more bend in the upper limb for 3 under than you need for split-finger.
Its not difficult to understand if you will think about what is happening. You want both limbs to be bent the same at FULL draw. With one finger above the arrow you will be bending the upper limb more than if all three finger were under the arrow, needing less tiller at braceheight. Now for the million dollar question, how much different? That can vary depending on how you grip the bow as that will also effect the bows tiller. If you shoot with a straight wrist ala-Asbell the fulcrum will be closer to the shelf causing a more even tiller then if you heal the bow, which will move the fulcrum down putting more bend in the lower limb which will require more positive tiller in the upper limb. I hope I didn't confuse you more.
Bob