There have been several threads with in depth discussions of the pros and cons of both styles, with many having tried both, and settled on one or the other for their own reasons.
The primary 'advantage' of the 3 under style is getting the arrow closer to your dominant eye to aid in aiming. Others, Fred Asbel for one, who adhere to an "instinctive" shooting style, maintain that since the arrow isn't used in aiming, placing it that close to the dominant eye, encourages the use of the arrow, thus interfering with true "instinctive" shooting. (See Asbel's DVD "Instinctive Shooting")They would all be proponents of 'split finger', which some say allows more control of the arrow.
For more of the member's input and opinions, use the 'search' function in this forum which should lead you to several discussions.
Enjoy!