Three under will close your gap, if you keep your anchor in generally the same place, just move the arrow on top of your index finger rather than under it. You need to nock below the top nock, and tie another one under the nock to keep the arrow from sliding down the string, whichever way you choose to shoot. There should be a small amount of space between the bottom nock and the arrow nock, to allow for the string angle that occurs at full draw, otherwise it could pinch the nock and cause problems. This gap doesn't need to be much at all, though, just some daylight. Almost all gap shooters use three under because it gets the arrow up under the eye and reduces the gaps. It lessens string pinch on the fingers, too, so you can get away with shorter bows if you like those.