Well, it's a complex matter. From an artistic perspective, if the lesbian threesome stories are used to show the diversity of human experiences and emotions, they could be considered appropriate in a sense. But from a more conservative view, they are often seen as inappropriate due to their sexual nature. The literary world is divided on this, with some publishers being more open - minded and others more hesitant to include such stories.
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.