A unicorn is a mythological creature that does not exist in real life!
That is why the phrase "looking for a unicorn" came to be used in the English language (not just poly circles) to mean someone who was seeking the impossible to find and has unrealistic dreams.
So, please don't say you are looking for a unicorn; that is actually an insult. You don't want to be pegged as unicorn hunters -- couples who seek single, bisexual women to "add to" their relationship, to move in with them before they even know her fully, and expect her to be sexually faithful to the couple only, and to love them both equally.
Of course, those are ridiculous expectations to lay on someone, and most unicorn hunters don't realize that, because they are so focused on what this hot bi babe will do for them, they forget that she will have a mind of her own, and individual needs, wants, desires, and baggage to be dealt with.
What can you do for her? How will being with you two benefit anyone? That is the question you need to ask yourselves! If it's just about sex, you are in the wrong place. Polyamory is about love and loving relationships.
Just date people, and don't insult them by thinking of them as unicorns, and don't expect nor demand that they have sex with both of you in threesomes only or cannot have separate relationships with each of you, or relationships with people other than the two of you. Date separately and if something comes out of it where you meet a woman you get along with who is interested in you bth, let it grow and develop organically.
People are not condiments, so you can't add them to your marriage like you would add ketchup and pickles to your hamburger. You can't have a role already planned for a person to fit into. You meet people, like you met anyone else you've been with in your life, and see if you hit it off.