From Halopedia, the Halo wiki
Members of this unit were involved in a skirmish in Sector Six, Asia during the Battle for Earth in 2552. A team from 3/1 Bravo was assigned to "recover" Master Chief by sighting his approach vector as he landed on Earth after arriving on the Forerunner ship. The team, consisting of at least five Marines and a Corpsman, came under heavy attack from a column of Brutes with air support from Banshees and Phantoms, however they were able to successfully hold off the enemy with the help of two ODSTs from Delta Team of 3rd Unit, 2nd Shock Troops Battalion. Though successful, the unit suffered at least one killed and two wounded during the fight.
It is not completely clear what "3/1 Bravo" stands for in terms of unit identification code. 3/1 Bravo could refer to Bravo Company of the 3rd Battalion, 1st Marines (Thus the unit would then belong to the 1st Marine Regiment.) However, the designation could also be far more specific, referring to Bravo team, first squad of the third platoon of an unnamed Company or higher organization. Unit designation codes have been used often in Halo but rarely enumerated upon. See UNSC military organization for further details.
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