Star Trek: The Lionhearted Wiki

Third Lieutenant, or 3LT, was a provisional rank used by the United Federation Marine Corps during wartime.

Civilians whose skills were in demand commissioned at the rank were given a pass on United Federation Marine Corps Officer Candidate School.  Normally these new officers served in non-front line positions, though pilots were the largest group brought in.

After six months they were eligible for promotion to Second Lieutenant.

The Starfleet equivalent was Ensign Junior Grade.