Once. It involved two friends and a quite a few drinks. I much prefer a threesome with a guy and another woman. Or just two other women.

Number one is topic. I gravitate towards NIP and ENF stories, and among those my favorites are stories about a single woman all alone engaging in risky public nudity. You know, where she gets completely naked somewhere she shouldn't. Work, school, library, park, neighborhood, indoors, outdoors, daytime, nighttime, doesn't matter. Nobody else is involved, nobody catches her and blackmails her, she's not being forced to do it, she doesn't end up having sex with anyone. Just her and her alone, completely head-to-toe naked and enjoying herself engaging in such risky behavior -- the riskier it gets the more aroused she gets. And while such stories do exist (excellent stories written by excellent authors) such stories tend to be hard to find which is unfortunate.
A short dress or skirt without panties? Short answer, no. But I have worn skirts at work (the shortest just below mid thigh) without panties, and on a few occasions if I'm wearing say a sweater or a business suit with a jacket I'll go without a bra as well. One time I was in a meeting where I was the only woman and I was completely commando and none of the men had any idea. If only they knew.
I especially like women getting completely head-to-toe naked in a public place, like the park, school, office, library, basically where they shouldn’t. There’s no one else involved, no sex (except with themselves), they don’t intentionally expose themselves to anyone. Just them getting aroused by being naked in public and the risk they could get caught. Very hot.
I was attending an after work gathering in a bar near my office. It was after a big project so I took my team out as a thank you for their hard work. It was fun, and while I wasn't crazy drunk I was feeling no pain. After most of my coworkers left I was hanging out with the two were still there. After a short while they left, and I stayed behind to settle our tab, but ended up staying longer when a group of 3 Japanese businessmen started chatting me up. They were friendly enough. After a few more drinks they invited me back to their office, which was a short walk from the bar. By this time my inhibitions were pretty damn diminished. On the elevator ride up one of them offered me $200 cash for my shoes. Since they didn't cost anywhere near that much and I was feeling frisky I took his offer. Long story short, they ending up buying all my clothes, which of course left me stark naked, head to toe, but about $3000 richer. Once we reached their floor, we exited the elevator, but they left all my clothes behind, save for my coat, bag and shoes. I made another $1500 sucking their cocks and and letting them blow their loads all over me while I masturbated (but no sex), then they got me a car service home. They were also nice enough to give me back my shoes, coat ad bag so I didn't have to go home completely nude.
Exhibitionism. In particular when a lone woman decides to get completely head-to-toe naked in a public place, indoors or outdoors. And by completely naked, I mean completely naked. Barefoot, no jewelry or accessories, nothing at all. Naked as the day she was born. She doesn't get caught and there's no sex involved (except the eventual sex with herself), but her fantasizing about possibly getting caught and the resulting ENF/humiliation only ends up getting her more and more aroused. Unfortunately such stories are hard to find.