I regret it a bit, but for weird reasons.
I went to a top 20 research school, went to Nationals multiple times.
A comparable school would have been an Ivy or Stanford. I knew some of the Stanford runners at the time. I knew a few Ivy guys too. I would have been fine in those programs. But I wasn't ready for their training at 18, but I knew a few guys on those rosters who weren't either. If I had been at Stanford, I'd have chosen something totally different as far as a career path, and I'd be in a different spot, but I have no real regrets about where I am.
I also don't regret my development. I was a state champ in HS, but on very low mileage. The school I selected was the best choice for me based on training status.
The thing is, if the question is this simple, it's too broad. I definitely do NOT regret going to some lower tier D1 just to say that I ran D1. I sort of regret not going to an Ivy or Stanford, but I'm not sure that's really a fair statement. So, on average, I'd have to say I did not regret going to a D3, because a lot of D1 schools are not as amazing as you think, either academically or athletically.
Having coached for a decade, I can tell you that Nick and Will don't have regrets about it either. The personalized attention that they received (that a lot of national caliber athletes receive in D3) was a huge factor in their success. Is it possible that either of them would have risen to the top sooner? Maybe. Maybe not. I'm not sure you can look back on Symmonds career and say, "he'd have been better at ___________". Hind sight is 20-20, but that's a hard statement to pull.