Today's Cerberus Vol. 1 (manga)

As a young boy, Chiaki Mikado had an encounter with an unusual, three-headed puppy, and as a consequence of a bite from that dog, he lost a piece of his soul. Now a teenager, he finds himself unable to enjoy anything, but the three-headed dog is back in human form -- or more accurately forms -- to fix all that!

Turns out that the three heads of the mythical guardian to the gates of the underworld are actually unique individuals, and each a pretty young girl at that! With his new roommate(s) leading the charge, can Chiaki experience the happiness that has so long eluded him?