Three Basic Orientations
The three orientations are an expression of the Law of Three, on which the entire Enneagram concept is based. This law states that there are three kinds of forces that act in the human nature - the Active force, the Responsive force and the Neutral force and that each person is born with a natural preference for one of them.
These three forces are similar to the Hornevian Groups (Assertive, Compliant and Withdrawn respectively), but they are used here in a different context, to describe inborn traits and parental styles rather than established personality.