Complex sentence: Originally the Three Stooges were signed to appear in the film, but they had just signed with Columbia Pictures for "Woman Haters", the first of their short subjects, so three look-alike actors replaced them in "Gift of Gab".
Split into simpler sentences: Originally the Three Stooges were signed to appear in the film. However, the Stooges had just signed with Columbia Pictures for "Woman Haters", the first of their short subjects, so three look-alike actors replaced them in "Gift of Gab".