"Kanojo x Kanojo x Kanojo - San Shimai to no DokiDoki Kyōdō Seikatsu" centers on the life of a young man named Tomohiro, who finds himself in an unusual living arrangement after his parents move abroad for work. Left alone in their large house, Tomohiro is soon joined by three sisters—Miu, Rika, and Aki—who are distant relatives. The sisters move in with him under the pretense of helping him manage the household, but their presence quickly complicates his daily life.

Miu, the eldest sister, is mature and responsible, often taking charge of the household. Rika, the middle sister, is energetic and playful, frequently teasing Tomohiro. Aki, the youngest, is shy and reserved, often relying on her sisters for support. As the four live together, their interactions grow increasingly intimate, leading to a series of romantic and comedic situations. The dynamic between Tomohiro and the sisters evolves as they navigate their shared living space, balancing their individual personalities and growing feelings for one another.

The narrative explores themes of cohabitation, familial bonds, and romantic tension, with a focus on the evolving relationships between Tomohiro and each of the sisters. The series blends lighthearted humor with moments of emotional depth, as the characters confront their feelings and the challenges of their unconventional living situation. The story progresses through a series of episodic encounters, highlighting the complexities of their relationships and the gradual development of mutual affection.