As has been said above, the main advantage to "three under" is getting the nock higher on your face and closer under your aiming eye. It should not change your draw length at all, as you can continue to anchor in the same position as shooting split finger. You're just moving the nock up by the width of your index finger.
I won't get into the "instinctive vs gap shooting" debate, but I will use "gap shooting" to explain what reducing the distance between your eye and the nock does for you.
While gravity begins to work on the arrow as soon as it is released, for some distance, the affect is negligible, and we say that the bow is shooting "flat" for that distance That's somewhere in the neighborhood of 15-18 yds, on the average. Consequently, in that yardage, the gap shooter will be holding the point of his arrow below the desired impact point the same distance that the nock is below his eye, whether it be 1.5", 2", or whatever.
As the shooting distance increases, that gap has do decrease, i.e., the arrow point has to be raised because gravity is having more affect over the longer distance to the target. At some distance, the point of the arrow is held right on the spot, and we call that the "point on" distance. This is frequently in the 30-40 yd range for a "3 under" shooter, and something like 60-70 yards for the split finger shooter, again, depending upon distance between the nock and the shooter's eye at anchor.
Since most ethical hunters are not taking shots longer than 20-30 yds max, the "3 under" gap shooter has much less room for error as his gap is so much smaller than that of the split finger shooter, given the same anchor point for both. The "instinctive" shooter also has the advantage of having to visualize a smaller range of arrow arcs than would the split finger shooter.
Don't hesitate to PM me, if you have any Qs about anything I've said.
Have Fun!