"The Trillium series is a series of five fantasy novels by Marion Zimmer Bradley, Julian May, and Andre Norton set in the World of the Three Moons. The series title is unofficial and is used solely because every novel of the series had the word "trillium" in its title. The concept of "trillium" or trinity (also a plant with a threefold flower: Trillium) plays an important role in the novels, probably because there were three lead characters and three authors working on the series. However, it should be noted that only the first book of the series, Black Trillium, was created by all three of them, while the other four were written by individual authors." -- Wikipedia