There shouldn't be enough of a spine issue to move the point of impact dramatically to the right when you switch to 3-under, if it would move it at all.
It's probably worth trying to figure out what the problem is, so you can avoid it in the future, whether you are shooting split or 3-under.
If your left misses are grouping well, other than the occasional ones that don't, it may be that you are holding your eye just to the left of the arrow, rather than over the arrow. Since the arrow is closer to your eye when shooting 3-under, you may be more aware of the arrow and keep your eye over it. The occasional misses that go way left could be due to any number of form issues, as Wapiti said, and are unlikely to be cured by switching to 3-under.
But you might try shooting some arrows split fingers and make sure your eye is over the arrow. If your eye is to the left of the arrow, the arrow will seem to fly to the target from the right to the left, but if your eye is over the arrow, it will seem to fly directly at the target.