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.