Billy Wilder, Ernest Lehman, Samuel A. Taylor
Three writers seems like a lot of writers; if too many cooks spoil the broth, won't three hands scritching at the same page spoil the screenplay?
Well, that might happen with three of your usual, run-of-the-mill writers, but Wilder, Lehman, and Taylor were something special. All three were very successful and admired—they rarely got their typewriters tangled.
Taylor was the writer of the original play Sabrina Fair, which opened on Broadway in 1953. He would go on to do an important rewrite on Alfred Hitchcock's Vertigo (1958).
Lehman is even more famous. Sabrina was at the beginning of his writing career, but he'd go on to write the screenplay adaptations for West Side Story (1961) and The Sound of Music (1965), as well as the famous Hitchcock film North by Northwest (1959).
And of course Billy Wilder was the director of the picture. Wilder had started out as a writer in the German film industry, before he fled to America during the rise of the Nazis. He was involved in the writing of virtually all his films, and Sabrina was no exception.
Still, even with all those experienced hands and typewriters, writing on Sabrina wasn't all soufflés and glamour. Wilder had a serious back problem, and as a result he and Lehman struggled to incorporate rewrites as the filming went on. At one point, Wilder asked Hepburn to feign illness so he'd have another day to get caught up on the script, and the executives wouldn't be mad at him. Hepburn went along with it, and did a good job of acting sick (because hey, she was a professional actor).
A good bit of the Wilder/Lehman/Taylor script was reused in the 1995 Sabrina remake, though a couple more writers were added too. The Larrabees have chauffeurs, gardeners, and every other kind of help. It makes sense they'd have a passel of writers on call to chronicle their doings as well.