So question....You take a shot at a 20 yard broadside deer..hit a little high and spine it. It starts to struggle to get up. You nock another arrow and let fly into the lungs/heart of the chest cavity. Do you sling a third arrow? If so, why?

I see no need once the vitals are hit. The deer will bleed out and die before you even get down and over to it...another arrow seems almost excessive to me. To each his own, but my bow hits the hook after the first shot to the vitals.