Matty, there are D3 coaches who will guarantee a roster spot. The guarantee is as good as the coach's word. The best thing to do is check out his reputation.
My son was guaranteed a roster spot his first year at a D3. He had mono and missed all of fall baseball. Did not make one practice due to illness. Players were cut from the team, but not my son. So I can say that the coach lived up to his word.
He is now entering his second year, and has to earn his spot.