I've been lucky enough to have been involved in two different threesomes over the years. Both instances involved a woman and another guy, rather than two women, my as yet unrealized goal. Interestingly, they both occurred within a single year.
This one involved a guy I'd known since high school (Tom) and an attractive woman who worked in the same office as me (Jan). Tom had always managed to attract his share of girls (and frankly some of my share too), but I didn't hold that against him. Over the years we talked freely about our 'exploits,' which is when we admitted that neither of us had ever been in a threesome. When I mentioned to him I had slept with Jan on several previous occasions and got the very clear sense that she would be interested in a threesome, Tom and I just looked at each other and said, 'let's go for it with Jan.'
It took a couple of days before the three of us were free on the same night. We went to a restaurant for a drink and settled in to one of their curved booths, with Tom and I on either side of Jan. Since I had already mentioned our little plan to Jan, we started in with some flirtatious banter, which soon became quite explicit once the drinking began in earnest.
Though we were sitting in a booth, there were a number of tables across from us at which some couples and guys were sitting and eating. The tables didn't have a tablecloth, so anything happening under the table was visible to anyone who looked our way. Knowing that Jan was quite the exhibitionist, I took the opportunity to slide my right hand into her lap and up the slit of her dress. Jan knew I was also turned on by her flashing other people, so she wore a wrap-around dress, held together with a belt, which meant it wasn't hard to find a way in. I guess she knew where I was headed as Jan turned to me, gave me a 'nasty' smile, and then continued on talking with Tom. I put my hand on her inner thigh and pulled her leg toward me just enough to spread her legs so that the folks sitting at the tables could see up her dress.
I had managed to do all of this without Tom noticing, but after a couple of minutes Jan managed to direct his attention to her lap. Seeing my hand on her thigh, he used his left hand to pull her other leg apart. So there we were: Jan's legs were wide apart and Tom and I had our hands on her upper thighs. Jan just kept drinking and laughing loudly which had the effect of getting the people opposite us to turn and look in our direction. It was almost as if she dared them to look up skirt. I noticed one or two of the guys looking and checking her out. Since we seemed to be putting on a show, I moved my hand up her leg, pushed aside her panties and put a finger inside her already very wet pussy. Well, I guess she felt I went too far, because she growled, turned and gave me a 'stop that right now' look, which I did. Her shout, of course, managed to draw the attention any other diners who hadn't already gotten a good view of her parted legs and her panties.
At that point Jan announced she wanted to go now, so I thought I had ruined all of our plans. I apologized for my actions, but she said forget it. The real reason she wanted to go was that all our playing around had made her so horny that she wanted to fuck the two of us ASAP. Well, Tom quickly paid the check and we drove back to Jan's apartment.