Relax the drawing forearm, draw the bow with your back and the arrow coming off the shelf issues disappear....relaxing the forearm cures many other shooting issues as well!
Not wanting to disagee...but IMO always use a second nock on the string, whether shooting 3 under or split...with approximately a 3/32" between the arrow nock and second nock.... reason being the arrow is nocked above actual center of the string, so as soon as the fingers clear the string, the arrow tends to move towards the center of the string.