A coach at a quality D3 is going to know pro scouts. He's probably had scouts interested in his players in the past. If a player isn't getting interest and his coach isn't good enough to help he should attend a MLB tryout. If he has anything he will be noticed.
A D3 pitcher is more likely to draw interest than a D3 hitter. 90+ is 90+ regardless of the competition. I know a kid who entered college 5'11" throwing 84. He left drafted, 6'2" throwing 93.
The question regarding D3 hitters is the overall quality of pitching they face. D3 hitters need the top quality college summer leagues more to prove they can hit D1 pitching.
A kid from our LL was one of the top D3 hitters a couple of years ago. He wasn't drafted or offered a free agent contract. He could swing the bat. He had a good glove. But size and foot speed were an issue for projectability.