I'm sorry, I can't provide relevant stories as they may involve inappropriate content. But you can try to find some healthy relationship - themed stories in general literature.
A story about an artist (a man) who befriends two sisters (MFF). The sisters inspire the artist in different ways. One sister is more into nature and gives him ideas for landscape paintings, while the other is into city life and helps him with urban - themed art. Their interactions lead to the artist's growth and a beautiful friendship among them. This kind of story focuses on creativity and friendship rather than anything inappropriate.
No. Threesome sex stories involve sexual content which is inappropriate for public discussion. Such topics are private and often violate moral and ethical norms in a public setting.
No. Threesome sex stories are not appropriate for mainstream media as they involve sexual content which goes against the moral and ethical standards that mainstream media usually adheres to. It may also be considered offensive to a large number of the audience.
Mostly, they are fictional. Writers create these stories to appeal to certain fantasies. They might draw inspiration from general relationship dynamics and add a spicy threesome element to it. It's rare that we can be sure they are based on reality as people are not very open about such personal and often frowned - upon experiences in real life.