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).
While the first two entries typically focus on the "setup" (the arrival, the initial awkwardness, and the first transgressions), is often where the narrative shifts from experimentation to deep emotional or physical attachment. shinseki no ko to otomari dakara 3