Prohibited inter-species procreation - eventually allowed, producing 4th gender. Two of the three genders are bureaucrats, thinkers/builders.
The Gods Themselves by Isaac Asimov
Published in 1972, the middle third of the book features an alien species from a parallel reality with three genders: Rationals, Emotionals, and Parentals. Reproduction among these aliens involves one of each type merging together into a single individual with a distinct personality.