A coworker (24F) is very open about her sexuality and her relationship with her boyfriend (26F). We’ve become good friends. She’s pretty, the perfect “thic” in my opinion, he’s tall / very fit / handsome so one day we got on the topic of 3-somes. She knew my wife and I (35M) have an open relationship, and I gave her some advice on 3-somes - how to prepare, do’s and dont’s, etc. I could tell she was intrigued. A couple days later, she invited me to her parent’s place during what was supposed to be a workday for a pool party with her boyfriend. She didn’t want to promise anything, but she made it known that this was an exploratory day for them. We had some drinks, laughed and had a good time the 3 of us, and I came to gather that he was definitely the dominant one in the relationship, and she was very submissive. He gave her a hard time about the original 1-piece bathing suit she had on saying it made her look like an old lady, and told her to go put on a specific blue bikini. She looked great, and her big natural DD’s sat perfectly on display for us. We hung out in the hot tub, he started loving up on her a bit and I could see they were getting aroused. He dropped his swimming trunks, and told her to put her head under the water for him. She went down briefly, then came back up for air. Again he said. She did it. Again. She did it. It was fucking hot and at this point I’m getting hard watching her get bossed around. Then he sits up on the edge of the hot tub, his cock is rock hard and about the same size as mine, and tells her to suck it again which she does. After what felt like a minute or 2, I work up the courage to sit up on the edge of the hot tub, clearly hard, and he tells her “now him”. I dropped my swim trunks, and he takes off her bikini top for me - probably the best natural tits I’ve ever seen - and she starts sucking my dick. He gets behind her, and starts giving it to her from behind as she blows me at the edge of the hot tub. It was so hot, and I could tell they were both loving it too. After a few minutes, it took all of my efforts not to cum, but thankfully her boyfriend was ready to, and he pulled out and shot a load across her back. I let myself relax, gave her the warning, and she got down slightly below me and took my cum all over her tits. It was absolutely incredible - hope to do a wife swap here soon with my wife!