I was published in a journal on Beall's list - how bad is my mistake? Please pardon my somewhat peculiar case, I hope this is still a question useful to others.

After school I became an undergrad but paused studying after a few semesters of university to work full time. During the several years I was employed, I also became co-author of three papers, two of which were were with a "potential, possible, or probable predatory scholarly open-access publisher".

At the time I was not aware journals like that even existed, and as a professor and two doctors were also authors, I would never have imagined this could be an issue. As I am now a university student again and learned about this, I obviously regret my decision to participate. I am not sure how my mistake will be judged by others, and how far-reaching the consequences are, this is why I am asking here.

To be honest, I think it, while bad, was no terrible blunder, as I was "only" an employee at the time (though an undergrad before that). Also, I suspect most future employers would probably not even recognize my mistake.

Clearly it is different if I were to pursue an academic career. Are past flaws of this kind forgivable - and if so, should I (actively) point out that I made a mistake and will avoid it in the future?

Annotation: Many people claimed (elsewhere) that the journal in question did no peer-review. This was not the case for me, as we always had a list of (content) issues to address, usually by 4 to 6 reviewers.
While publishing in a predatory journal doesn't help your CV, I don't see this as anything near a fatal mistake even if you are pursuing a career in academia. The main cost is just the wasted time and effort and the fact that you could have gotten credit for these papers if published elsewhere.

Given that you were at such an early career stage when you wrote these papers, it seems that you can readily be forgiven for falling victim to a predator publisher. Tricking people in this way is, after all, often a component of their business model. When I see a professor publishing repeatedly in these journals, that suggests he or she is trying to pull something over on someone. When I see an undergrad or someone straight out of undergrad doing so, I feel badly for the student and angry at the publisher.

I would simply list these papers on your CV and move on. They're part of your publication record, so you can't really omit them in good faith -- but nor do you need to beat yourself up over it. If someone asks, be forthcoming both about the fact that this journal is questionable and about the way that you ended up publishing there. 

You haven't done anything terrible. You've been exploited, and maybe if someone is a harsh judge he will think this makes you look a little silly. But in your situation I don't think most people would hold that against you.