I played around a bit with three under a couple of years ago. I'm no expert by any means, but my thoughts were that it seemed to be more accurate at close known yardages, but I didn't seem to be as good at dropping them in from unknown yardages. Might just be me, but it didn't seem like I was as good at judging distance instictively with three under.
Also, it did sure seem to make the bow louder for whatever reason. Maybe because my bows are tillered for split finger, or maybe I needed to play around more with the location of string silencers. I didn't stick with it long enough to get comfortable with it.