Madeleine L'Engle and her family were traveling from rural Connecticut to New York City in 1959 when suddenly she thought of three character names: Mrs. Whatsit, Mrs. Who, Mrs. Which. The story of A Wrinkle in Time began falling into place almost directly, until it became, as L'Engle described it, 'a psalm of praise to life'. L'Engle had considerable difficulty finding a publisher, however; publishers were baffled by a book that seemed to use the tropes of a children's book but was so much more advanced than their conceptions of a children's book. The literary agent L'Engle had hired actually gave up on it and returned the manuscript. So it would all have ended, except that L'Engle threw a small Christmas tea party for her mother and one of the guests, hearing the story, connected her to the publisher John C. Farrar. Farrar did not publish children's books at all -- but he liked the story and agreed to publish anyway. It was an instant success.
In 1970, L'Engle wrote a short story in which Mrs. Whatsit, Mrs. Who, and Mrs. Which send Meg, Calvin, and Charles Wallace to a school on another planet; it was in a sense just a quick bit, but as it intersected reading about mitochondria, which raised the idea of a work not about the macrocosm, like Wrinkle, but about the microcosm, it became A Wind in the Door. The third work in the series, A Swiftly Tilting Planet, followed in 1978.
There are two other works that are often bracketed with the first three, Many Waters and An Acceptable Time. I don't have An Acceptable Time -- which gives a story of a later generation -- but I do have Many Waters, so I'll add that to the mix, to make the 'Time Quartet', as it is called, the reading for the next fortnightly book.