Defined as a polyamorous relationship/union with exactly three people. In a true triad, all members are sexually and emotionally involved with every other member. The most common formula for a triad consists of one woman and two bisexual men or one man and two bisexual women.
Kevin is heterosexual, and Linda and Beth are bisexual. He is attracted to both of them, and they’re both attracted to him as well as each other.