You say you are newly single. I don't know how new, but your son is probably reacting in his own three year old way to his life being turned upside down. He doesn't understand grown up problems, but he does understand a stressed out mom and a suddenly gone dad.
Make sure that you are spending as much time with him as you can. I have found that my kids open up to me most when we are doing something they want to do -- walk to the park, read a book, play pretend, etc. I'm sure you may not have a lot of energy, but it is important.
As for the stripping specifically, reinforce that taking his clothes off in public is wrong. Have a consequence and stick with it. If this is a habitual problem, you could offer a reward for a week (or a day) without stripping such as ice cream for dessert or getting to watch a special show.
You didn't mention if he was potty trained or potty training. That could also be a factor that you might want to consider.