Good question. I have written a few threesomes, Throne of Shame and Lupercalia spring to mind, but they’re quite unconventional trios. I think this is because writing with three introduces the challenge of establishing the third character, and in particular, what they’re actually doing in the story.
I personally think it’s important for erotic stories to be believable. Most sexual relations involve two consenting adults in private – so if you want to break that convention your story needs to justify it, otherwise it’s going to seem very contrived. Everyone knows the classic porn cliche – oh look! her sister/best friend/co-worker is here, and she’s horny too! Lame contrived settings are one reason why most porn movies are ultimately so unsatisfying.
So yes, I expect I will occasionally write with multiple characters, but only if I can imagine a setting that does the characters justice.
Thanks for the question!