MK, very thoughtful response!
It can take only one event to be seen. Having been through this process twice with HS players and now again with a college transfer, I wouldn't bet on it for the majority of players including my guys. But the question was about attending one event...
For the vast majority of players, it's about being seen live by the schools on that player's vetted list. That can happen at a single event. Pitchers, which are about half of all recruits, will generally be watched by most coaches at an event. Every team needs lots of pitchers and I would guess that for D3 level talent, it's possible that one event could do it. It's a little more challenging for position only players IMHO.
Since the question asked about D3 players, I will add that in my experience, demonstrated interest in the school (baseball and admissions) is a big piece of the equation. A D3 player is not being offered athletic money. Many coaches have little to no pull with admissions. A student athlete is wise to connect with their admissions rep (e.g., fill out the online admissions profile, interview, meet on campus or at the player's high school, tour campus, etc.) in addition to connecting with the baseball staff (e.g., online profile, sending video, emailing updates, calling, etc.).
Doing all the things necessary in advance of showcasing helps with the process whether it's a one showcase deal or whether the player is ultimately doing several...or more.
And regarding the original topic of the Stanford Camp, given the list of schools attending this coming Summer (which in my experience is similar to previous years) there are not a lot of D3 schools relative to a HF.