My Girlfriend's Not Here Today Vol. 3 (manga)

Yuni went to Osaka to cheer on Nanase at her big game, but she ended up in bed with Fuuko. Even though Yuni insists to herself that this is mostly Nanase’s fault, she still feels terrible. Back home, she’s invited to a group outing at the pool.

Her heart leaps at the idea of a swimsuit date with Nanase, but then drops when Fuuko tags along too. The deception is getting harder and harder to keep up, especially as Nanase starts to question Yuni’s friendship with Fuuko.