A third officer is an officer of a starship that is the fourth in line of command of the vessel after the captain. In Starfleet, the position is generally filled by a lieutenant commander or lieutenant. The Security chief or Tactical officer is sometimes given this position.
Known third officers
Though Paris has never explicitly been named third officer in canon, in VOY: "Unimatrix Zero", Paris comments that, with Janeway and Tuvok away and Chakotay in command, he (Paris) is now Chakotay's first officer. It can be inferred from the above dialogue that he was the third officer, despite the presence of higher-ranking non-bridge personnel on Voyager.
Data was second officer of the Enterprise-E, but Deanna Troi had a higher rank than him, and was the highest ranking bridge officer after Picard and Riker. Troi is in command of the bridge while the Enterprise-E is traveling to the Briar Patch, and when Data leaves the bridge to head to the Scimitar.
See also
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