Bowyers differ on this, but many 3 under bows are tillered at 0. Many split finger bows are tillered +3/16” (more or less). Since drawing the bow 3 under puts the drawing hand a little lower on the string than drawing split, a common adjustment is to raise the nock point a little when a bow tillered for split fingers is drawn 3 under, which brings the drawing hand a little closer to where it would be if the bow were drawn split.
If you draw a bow split fingers that is tillered for 3 under, you are putting drawing hand a little higher than the bowyer intended. Never having tried this, I don't know if this would call for a slightly lower nock point, although logic would say that this might be the case. Of course, proof of the pudding is whatever it takes to make the bare shaft fly level.