The most valuable thing D3 coaches can offer is a chit through admissions, since there's no athletic money available. Second most valuable thing is a chance to play in college while getting a solid education.
If you're on the short list at a D3, coach will ask for your grades, test scores and senior year courses in June. They will submit this to admissions for a pre-read usually in late June-late July (every school has a slightly different timeframe).
So you'll want to be on the HC's preferred list by mid-June before senior year. A current HS junior should reach out to the HC at the schools he's interested in ASAP. Keep the email short. Send them short videos. Keep them up on the player's progress through Fall, and send the fall/winter/summer schedule if you have it. If you can afford it, visit campus and attend one of their camps.
If you get a positive pre-read then you'll be encouraged to apply ED1, deadline Nov 1st, response usually by Dec 15.
The admissions chit is more valuable the higher up the academic chain you go. Many players only make it into a specific college because of athletics.
FWIW - my daughter's boyfriend is a pitcher at a HA D3. He told her that D3 is made up of pitchers who couldn't throw above 90 as a HS senior. This guy is 6'5". He threw 88 as a HS senior and 92 as a college freshman. He's still at the D3.
Competition to play in college is fierce. D3 ball is no joke.