why are there three principals in the royal arch instead of just one

Under the English system, a Chapter is governed conjointly by the Three Principals—King, Prophet, and Priest—who represent Zerubbabel, Haggai, and Joshua 2.

Walter Leslie Wilmshurst interprets this triune leadership as a representation of the human spirit in its complete, regenerated state 1. In his reading, the three officers embody the spiritual attributes expressed in the very title of the Order: Holy, Royal, and Supreme (or Arch) 1. Rather than operating as distinct authorities, the three act conjointly as a single unified unit, mirroring the trinitarian structure of the spiritual principle in man 1.

Zerubbabel serves as the central governing term, personifying Royal Supremacy—the directive majesty and central power of the spirit 1. From this central term differentiate two complementary aspects: a passive aspect and an active aspect 1. Haggai, the Prophet, personifies the passive, self-contemplative aspect of spirit; he represents holiness, spiritual insight, and divine intuition 1. Joshua, the High Priest, personifies the active, executive aspect; he represents functional power and acts as a mediatorial force capable of subduing and transforming man's lower nature 1.

Together, the three officers depict the summit of human consciousness—royal as a child of Deity, holy in its spiritual perfection, and supreme in its power to align the lower self with the higher 1.