Some high schools have a sort of ceremonial piece of paper the D3 kids can sign, but as JCG said, there is no signing in D3 because there is no athletic money.
Not quite complete as D3's can have an NLI.
Here is a link to Informed Athlete (Rick's site) which explains a somewhat recent change bringing in a D3 NLI:
http://informedathlete.com/nca...on-iii-rule-changes/
There are now D3 NLI's but very different than D1/D2.
JCG really has it right, I think.
There are many variations in the recruiting and commitment process at the D3 level, perhaps as many as there are baseball programs. Our son's former coach makes it very clear to a recruit when that player is one of his "recruits." Unless someone is accepted on ED, even making a deposit after acceptance may not be"binding" if the family can afford the financial aspects (some D3 recruits get D1/D2 opportunities during the Summer after their senior year.)
My personal view is D3 recruiting is quite a bit different than the current form of D1. Every student athlete should visit the schools and, if they have baseball in the equation, watch games/practices, stay with players, meet with coaches, etc. Potential recruits need to listen closely to what the coaches are saying about his ability and his potential as a recruit.
So many on the way up are seeking some type of "guarantee" or at least "assurance" of a roster spot. At the core level, even an NLI for D1/D2 and even with 4 year guarantees at Power 5, there are no "guarantees" in college sports and especially college baseball.
Competing on a college baseball team can be second to none for athletic experiences. On the other hand, probably an equal number (including those who sign NLI's) have a "subpar" experience as those who have the second to none experience. College baseball is about competing, performing and producing. Nothing which occurs before a player steps on a college baseball field is very meaningful in terms of success in college baseball (including an NLI.)