I've had many 3sums over the years, both MFM and FMF. The first one was in college when I was working at a bar with a good friend and my gf. My buddy was jokingly referred to Perfect Bob. All the girls loved him and probably some of the guys did too. One night we were all there drinking on our night off. He was very drunk and made some random comment about my gf being hot. I told her, she knew I had been wanting a 3 sum with another girl and her. She said, if you want that, I want a 3 sum with you and Perfect Bob. I asked him if he was interested, and his response was hell yea. We ended going back to my place. It wasn't awkward at all. We all had a great time. Learned that even naked he was perfect, huge cut cock, but I'm pretty well hung, so it was cool. She had orgasm after orgasm. Only happened once with him. (Another positive thing about this experience: after this she owed me the FMF 3sum and gave it to me on my birthday)
Had a few more during college, but few years after I was dating a very adventurous girl. She was always wet and horny. My best friend at the time had a huge crush on her. One night we were drinking at our place and during a regular movie (R-rated) there was a MFM threesome. She joked that she would be down for that. She knew I'd had them before, so we both looked at him. He asked if we were serious. Next thing we know we're all naked and heading to the bedroom. It ended up being a yearlong relationship. He moved in with us. The 3 of us would sleep together every night, with her in the middle. It was great and totally straight except for one minor thing. I mentioned that in one of my FMF threesomes one girl would rub my balls while I was fucking the other and it helped me cum. After that when I was fucking my gf he would occasionally rub my balls....she never knew, and he and I never discussed it. I really think he did it to make me cum faster so he could fuck her, but whatever the reason, it felt great. It lasted for a little over a year until he met a girl he wanted to date.
There were many more over the years, but one of the more interesting ones occurred when a buddies ex gf invited me over to have dinner with her and her husband who was a cop. (She and I had become friends when she was dating my buddy and we stayed friends after they broke up) I was sitting on the couch having drinks while they were a few feet away in the kitchen finishing cooking the meal. I noticed that he came up behind her and was kissing her neck. It looked like he was rubbing her breast, but I wasn't sure. I joked "Should I leave?"....He spun her around and lifted her shirt and said, aren't these magnificent? All I could say was "beautiful" One thing led to another, and this was the best part. She was into DP...she loved a cock in her ass and the other in her pussy, but her favorite was two cocks in her pussy at the same time. I had never done that but rest it assured I loved it. Felt amazing and he always came before me..between the warmth of his cum and the juiciness of her pussy, I came soon too. We all played for about 6 months until she got pregnant. To this day, I wonder if I was being used to get her pregnant. The child resembles me in many ways, but I've never had the nerve to ask them directly.
Since this thread was about sharing a girl with a bud, I haven't included any of my FMF stories, but they are equally as exciting.