Yes threesomes happen, maybe not exactly like they do in porn.
When I was at college or uni I thought of stuff like threesomes as just a far out fantasy but one thing I learned as I got older, was that actually people are more sexually free when they are out of the academic environment where everyone knows everyones business and people think/talk about sex all the time. When you're older and out of that people just do their own thing. Some people settle down and have monogamous relationships, some people live quite sexually liberal lifestyles and keep it private. It's not glamourous and not everyone is a super hot model but there are normal people out there who live normal lives who experiment sexually and to them its not a big deal.
So what I found was, sometimes I just found myself in situations with those kind of people and I had experiences I never expected.
I had one in a houseshare where I lived with a couple. We all got on well. The girl in the couple was very sexually open and bisexual and we had a lot of open conversations. We used to talk a lot and have deep conversations about life, relationships, sex and I'd talk to her about situations I was in with other girls and ask her advice etc. One night the three of us were sat up drinking having a deep conversation and somewhere in it the conversation came up about threesomes and would you be comfortable, and the couple basically said yes and we've had them and she'd been in a relationship with a girl before and the other girl's male ex had done stuff with them. I asked about would they do it again and they said yes if its with someone we knew and trusted and I speculatively half-jokingly asked if they'd do one with me and they said yes.
With threesomes there are rules about what people are comfortable with, especially if there's a relationship involved in the middle. This is where it can be different from porn. With us, it was basically that the couple had sex and the girl pleasured me. I wasn't allowed to have sex with her and I wasn't allowed to pleasure her (that was the guy's rules). Or the guy would watch her pleasuring me while he entertained himself and then he would have sex with her.
We did variations on that arrangement I think about five times over about 3 months. It made for a strange atmosphere in the house as it felt like there were three of us in the relationship, and the girl and I were quite touchy-feely like would hug a lot and I'd stroke her hair on the sofa and that was all cool. I regret one thing which was one time when the guy was away me and the girl were having some banter and I said something suggestive that she should do it to me then and she just went "tut tut I thought you were better than that" and after that we didn't do it again and things were always a bit more formal between us, which was a shame.
I don't regret it, it was all fun, but honestly I do regret the end bit because I felt like I had a very close and easy friendship with the girl and I never got that back and we don't really contact now.