Anyone shoot split finger on a bow tillered three under? Or vice versa?

I think just adjusting your nocking point to accommodate whether you shoot split fingers or 3 under should do the trick.

My experience is that it works fine 80% or more of the time. There have been a few times I haven't been happy with the results shooting 3 under on a bow tillered for split. Never tried it the other way. One thing to try if a bow doesn't tune the way you want as a last resort before giving up on it would be to swap the upper and lower limbs, if they are reversible. I have had cases where I think the bowyer may have mismarked the upper and lower limbs.

Just curious, I shoot split fingers and I see many bows for sale tillered 3 under. Usually I don't consider them, maybe I should rethink that option.

Bowyers differ on this, but many 3 under bows are tillered at 0.  Many split finger bows are tillered +3/16" (more or less).  Since drawing the bow 3 under puts the drawing hand a little lower on the string than drawing split, a common adjustment is to raise the nock point a little when a bow tillered for split fingers is drawn 3 under, which brings the drawing hand a little closer to where it would be if the bow were drawn split. 

If you draw a bow split fingers that is tillered for 3 under, you are putting drawing hand a little higher than the bowyer intended.  Never having tried this, I don't know if this would call for a slightly lower nock point, although logic would say that this might be the case.  Of course, proof of the pudding is whatever it takes to make the bare shaft fly level.