A unicorn in polyamory or swinging lifestyles is usually a single bisexual willing to have sex or enter a relationship with an established couple. Although unicorns are typically female, this term can apply to any gender. Because this type of person is hard to find, they are similar to the mythological unicorn. They are in high demand, so this puts you in a position where there are plenty of offers and can choose the kind of relationship you want.
Being a unicorn in a triad does not mean you are a secondary. A triad means equal. As in any connection, there needs to be healthy boundaries, communication, and respect. A triad is a partnership. The couple may have rules designed, make sure they work for you and do not hesitate to use your voice. If a compromise is not possible, that means this couple is not right for you.
There is no rush, no need to jump into a triad, no matter how excited you are to get started. Let the couple wine and dine you, learn about them, check out their home, consider this a trial period. Ideally, the arrangement is not temporary, make sure you have good chemistry with both people and feel comfortable.
It is crucial to discuss dynamics: Who is dominant? Sexual protection? Hard and soft limits?
Remember, you are a magical, mythical beautiful unicorn. You are the one who holds power. Do not allow yourself to be disposable. Please make sure you want them in the same way they want you.
A good note: If you are new to the BDSM community and are interested in becoming a unicorn, check out this article https://www.autostraddle.com/to-unicorns-from-an-ex-unicorn-287425/