Because it's not a true threesome unless all parties are mutually attracted to each other. Bonus if it's not treated like a one-night stand but more of a forever thing. Double bonus if it actually is a forever thing; Brona becomes an undead immortal like Caliban and Ethan doesn't age like humans do (assuming he is a werewolf) and Dorian is totally into that.
Ethan/Brona/Dorian: Polyamory