Third officer

&nbsp The third officer, or 3O, is the fourth-in-command onboard a starship or starbase, following the commanding officer, first officer, and second officer. If the 3 senior officers are killed or incapacitated, the third officer will assume command as acting captain. The third officer role is usually combined with the role of security chief, chief tactical officer, or chief engineer and generally holds the rank of lieutenant commander or lieutenant.