The joke is that after Futakomori, it was obvious that Hell Girl's third season would get a title with "three" in it, and Mitsudomoe happens to contain "three" already. In fact, the real title turned out to be Mitsuganae.
Continuing from Hell Girl and Hell Girl: Two Mirrors, we come to Hell Girl Season 3. The title predictions are too cliche, so I don't want to say them.
