The reflex answer to that question is, "the 94% of high school varsity players who do not play college baseball." But I acknowledge that some of those players could play, but choose not to.
The more complete answer is that, yes, there are some D3 programs where just about anyone could make the team. Of course, to do so, you first have to get into a real hard school (e.g., Cal Tech) or be willing to go to a school in the middle of nowhere (e.g., Maine-Presque Isle) or feel comfortable at a small school with a heavy religious emphasis.
However, if you're talking about baseball at a reasonably competitive D3, the answer to "who couldn't really?" quickly gets back a lot closer to that 94% of high school varsity baseball players. And if you're talking about the D3's that contend for the postseason, the answer is an even higher percentage of high school players who really couldn't.