Unicorn hunters are very common. A person can't really "be" a unicorn despite that some bisexual women and the occasional bisexual man may think they "are" one. A unicorn in this context is simply an idealistic notion that these couples have that they can decide exactly what they want and then search for another person to fill that script, instead of meeting people first and THEN deciding how they might fit into the couple's life, WITH the third person's input of course. Because the unicorn really only exists in the minds of the couple, it is impossible for a real person to "be" a unicorn.
There are plenty of bisexual women who are into dating couples, or open to it if the right people come along. The slang term for them i hear used most often is "hot bi babe" or "HBB" for short. Of course, a woman can be bisexual and involved with a couple or open to it and be neither hot nor a babe, but those two things are subjective anyway and i'm not trying to debate the meanings of the words "hot" and "babe" in this context. Someone else might like to, though.