X2 on what everybody has said. If you left the nock where it was for split, the force of your draw would be exerted lower on the string when drawing 3 under, thus bending the lower limb relatively more than it bent with split. The objective is to have the limbs recover from the shot at the same time. If they were recovering at the same time when you shot split, the upper limb would recover quicker if you left the nock in the same place for 3 under.
However, you don't really need this theory. What you need to do is to shoot a bare shaft and adjust the nock until you get level to slightly nock high flight. If you do this, you will probably see that the nock will have to be placed 1/8" to 1/4" higher than it was when shooting split to get the same results.
If you have a good release, you will also be able to see that if you put most of the pressure on the index and middle fingers, you will be able to achieve level or slightly nock high bare shaft flight with less of an upward adjustment than you would if you exerted more equal pressure with all three fingers. Gil puts tape on his ring finger. I consciously hook my index finger and just rest the other two fingers on the string prior to beginning my draw. During the draw, the other two fingers take some weight, but not as much as they would if I hooked all three fingers prior to drawing the bow. I also think this results in a quieter bow. I don't like having a nock point 7/8" above square. Whether this makes any real difference or not, I don't know, but it bugs me. Getting the pressure off the ring finger has enabled me to drop a 7/8" nock point to 5/8", for some bows I have tested.
I have never been able to get reliable bare shaft results for 3 under unless I have a nock point above and below the arrow. I use tied on nocks, because the thought of having two metal nocks on the string scares me, and I have enough mental problems shooting the bow without adding any more.