A bow tillered for three under has a little less tiller in the lower limb than the upper limb. Or, saying that another way, the lower limb is a little stiffer. It's stiffer on a bow tillered for split finger as well, but three-finger under tiller is even more stiff by comparison. Theoretically, moving the nocking point up on a bow that is not tillered for three fingers under would shift a little of the draw force toward the upper limb, in effect weakening it and stiffening the lower limb The amount of change would likely be very small and perhaps unmeasurable.
In fact, there are a lot of folks shooting three under with bows that weren't tillered for three under. I would guess that's the case with most three-under shooters. And they get very good arrow flight. A bow tillered for three under should be a little easier to tune for three under than one that's not. That's about the only difference.