the main shift 'tween split and three is which string finger gets the lion's share of pressure. for split, it's the middle finger and with three it's the pointer finger. in both cases it's important to maintain most of the string holding pressure on the finger that's under the arrow nock. and in all cases, the pinky just about goes along for the ride ... which brings up the subject of the 2 finger flemish string grip. :cool:
addendum - it should be noted that the middle finger lies perfectly in line with the forearm bone, whilst the pointer finger is slightly offset. so, with 3under, you need to slightly torque the hand for optimum form alignment. more than a few world class olympic fita recurve archers shoot split finger, but remove the pointer finger off the string, which is in effect a flemish string grip!