recently swithched from split finger to 3 under.  much better groups but now hitting left. when I go back to split this goes away. any thoughts on how to correct this?

Use a thinner side plate.

Without seeing you shoot, I'll guess dragging your ring finger during your release.  If you don't have a high drawing elbow, I would concentrate on a deep hook and I would make sure my middle finger has at least 1/3 of the load.  The problem with 3 under is to let the top and bottom finger do all the work.  This results in torque.  Bill

The simplest explanation maybe that with 3 under your anchor may have moved slightly to the outside.

IM with Bill, it might be dragging your ring finger on the release.. Occasionally I totally massacre a shot when my ring catches and throws the arrow either way high or way left

I shoot 3 under and when I first switched I was dragging my ring finger alot. Once I started putting more pressure on the middle finger the leftys went away.

ceo hit the nail on the head

I can say that I Agree with CEO because that is exactly what has happened to me. I am a right hander and I was hitting left and i just didn't get it.  The I started holding for an extended period and looking at the arrow instead of just the tip.  The tip was on the target but the anchor was slightly to the right of my eye so that the arrow was actually pointing right where I was hitting.  I needed to re-align my shot by taking a little off of my head cant to bring my eye back over the arrow.  I still mis-align but if I hold long enough it all becomes apparent.  Time and repetition are the cures.

Rick

What is the reason why you have gone from split finger to 3 under?

Can you confirm if your reference point has changed between the switiching of styles?

I changed to 3 under, with the same reference point, to get my eye closer to the arrow. This did tighten my groups. I later cured the shooting left problem with the blank bale. On the blank bale I concentrated on reaching a firm reference point and clean release,,, a couple of sessions and the problem just went away.

Cant the bow more, if you don't want to change anything else. This moves the impact to the right for a RH shooter.