You got into very good schools, it doesn’t make sense to take a gap year. Rankings aren’t everything. I think Northeastern or Richmond would provide the best experience, but obviously it depends on you. Richmond is in a small city and is a liberal arts college; a very different experience from Northeastern which is in a bigger city and is a research university.
Richmond is probably more prestigious and has better academics.
I’d go with Northeastern because I’m an international student and Northeastern apparently has many. It’s also in Boston and I love big cities. Richmond seems to have more parties and sport events, which I don’t enjoy. I would be giving up the prestige and quality of Richmond, but it’s not enough of a difference that I am willing to give up Northeastern’s pros. You could be completely different from me and prefer Richmond.
Oh and co-op is also a huge thing at Northeastern. It usually takes 1 semester or year more to graduate (but I don’t think you pay for the time you do co-op.) It seems like you would enjoy that. Personally I don’t see it as something very special but that’s just my opinion.
So it’s up to you.
Good luck and congratulations!