Justin,
I can only offer some things to consider. First, you may be getting a cleaner release using 3-under. Second, are you sure the arrows are faster or is it that the 3-under puts them closer to your eye and the appearance of increased speed is an illusion? If you don't have a chrono you can do a rough estimate of relative speed based on depth of penetration into a target of consistent density. A hay bale or fairly new bag target will work. The increase in draw length is a real head-scratcher but may also account for the increase in speed. The "why" is the question. Last night I tried drawing split-finger and 3-under and measuring my draw length with each. They were the same. The only thing I can think of is to ask if you are bending your fingers more when shooting 3-under. Your fingers should be nothing more than a hook attached to the string with the back of your hand flat from the wrist to the second knuckles. If you are pulling the string by curling your fingers as you draw that would account for the increase in draw length. Just a guess on my part and someone else may have a better answer.
John