Which of these groups of siblings from children's literature is a threesome and not a foursome?

There are only 3 Waterbury children in Edith Nesbit's novel of that name which was first published in book form in 1906, although it had, in common with many other books at the time, been previously published in serial form. Their names are Roberta (generally known as Bobbie) Peter, and Phyllis.

The others are all groups of 4 siblings - respectively Mary, Laura, Carrie and Grace (Little House on the Prairie); Meg, Jo, Beth and Amy (Little Women), and Peter, Susan, Edmund and Lucy (The Lion, the Witch and the Wardrobe).

"The Railway Children" was made into a highly successful movie in 1970, and marked the directorial debut of Lionel Jeffries. An interesting fact is that although Bobbie in the book (whose heart-rending cry of "Daddy, my Daddy" has become one of the most-quoted lines ever) was several years older than Phyllis, the actress who portrayed her, Jennie Agutter was, in fact, younger than Sally Thomsett who played Phyllis.