Definitely not. These kinds of stories are not only immoral but also can have a negative impact on society's perception of family. A family should be a place of love, care, and positive values, not associated with such improper and unethical ideas.
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.