Here is a thought. When you shoot split finger, you are used to getting the string hand in tighter on your face and consequently the arrow nock is in tighter. It would seem that with 3 under, you may be getting your hand to the same place but the arrow nock is higher on your face and so is out farther than you are used to. It may be higher on your cheek bone which has moved it out to the left for you. Another way to say it is that the arrow nock is not as under your eye due to the cheek bone as with split finger.
Take a look at that in a mirror and see.