From Halopedia, the Halo wiki
Petty Officer Third Class, abbreviated to PO3, is a enlisted rank in the United Nations Space Command Naval Command (NAVCOM) rank structure; including the UNSC Navy and Office of Naval Intelligence. It is preceded by Crewman, and subordinate to a Petty Officer Second Class.[1]
Work party lead.[1]
Enlisted: Crewman Recruit • Crewman Apprentice • Crewman • Petty Officer Third Class • Petty Officer Second Class • Petty Officer First Class • Chief Petty Officer • Senior Chief Petty Officer • Master Chief Petty Officer
Enlisted: Private • Private First Class • Lance Corporal • Corporal • Sergeant • Staff Sergeant • Gunnery Sergeant • Master Sergeant • First Sergeant • Master Gunnery Sergeant • Sergeant Major