There are many fine archers who get good groups shooting either way. Getting good groups depends on having well tuned equipment and consistently good form. If you're a beginner, you should try to get some feedback on your form, either through a class, a coach, or by sending in a video to this forum.
Instinctive shooters, those who do not use the arrow or sights as an aiming method, tend to prefer split fingers.
Many shooters who aim using the tip of the arrow prefer 3 under, while others prefer split fingers.
So I would say the decision on whether to use split fingers or 3 under should primarily be based on whether you want to be an instinctive shooter or use an aiming method.