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.