The phrase (roughly translating to "Because I'm staying over with my relative's child 3") appears to refer to the third volume or entry in a Japanese adult media series (manga, anime, or game).
Here's a sample post:
Since "Shinseki no Ko to Otomari Dakara 3" translates roughly to "Staying Over with My Relative's Kid 3," this appears to be a request for a write-up on a specific entry in a niche mature-themed series (likely an adult visual novel, doujin, or manga volume). shinseki no ko to otomari dakara 3
"Shinseki no Ko to Otomari Dakara 3" or "The Young Lady and the Shinsengumi 3" is a Japanese manga series written and illustrated by Chie Shinohara. The series is a historical romance and comedy that revolves around the life of Chika, a young woman who becomes involved with the Shinsengumi, a special police force in Kyoto during the late Edo period. " Shinseki no Ko to Otomari dakara 3
— possibly a short doujinshi, ASMR audio title, or indie game, but without more context, it’s not identifiable. Weaknesses: Here's a sample post: Since "Shinseki no