I don't think there are 2-3 set places like you hear about with Headfirst and some of the other academic showcases but I did see several D3 schools last year at Baseball Heaven in NY and Diamond Nation in NJ at tournaments - more towards the end of the summer as the D1 recruiting seemed to be dying down. We also played at a tournament around Boston over the summer that was held predominately at D3 schools and I heard there were coaches there at some of the games - although 2016 was a PO and there did not appear to be many schools at the game he pitched. So you will find them out over the summer at these tournaments but it's kind of hit or miss.
From what we saw the D3 schools do a lot of recruiting at their own "prospect" camps. Most of those were late summer and then they really pick up in the Fall. Several coaches at Headfirst told me they recruited mainly at HF on Long Island and then at their own camp in September. We visited 2016s centennial conf school the weekend of their prospect camp - he didn't attend but they were talking about it like they were very excited to see several of the players in person.
Then there are a lot of regional showcases that can range from 15 to 25 schools in attendance. If things had not worked out at Headfirst like they did for 2016 we were going to attend several camps and several showcases but only the ones that had 1-2 of schools on his list. I'm talking strictly northeast - I saw several in PA and NJ and PBR showcases in NY and MA. He was focused on academic schools but to me they were more mid-level academic schools. The showcases I looked at seemed to have a mix - we never even ventured upstate NY but I know from emails I received that there were several showcases up there with a good mix of schools (D1, 2 and 3). A good place to look for the showcases is at your local baseball training center or through the top travel organizations in your area. I know three of the organizations locally held college day workouts at their facilities over the winter.
You will probably have to look around to find a showcase with the right mix of schools but if you target specific D3s you can definitely find out when their camp is, contact the coaches and attend. You could also find out what other camps/showcases they attend by asking.
And finally there is dumb luck. 2016 pitched at Diamond Nation mid-summer. They switched his schedule around to have him and another pitcher throw the early (8am) game against a very good team because they (his coaches) figured several schools would be there to watch the other team. It was a total pain to get there at 7ish instead of noon for the late game but they were right. I would say 2016 pitched just OK but the coach of that travel team is also an assistant coach at the school he'll attend and that got him on their radar.
Sorry for the long post.