Sol Defense Group 3/A-5

Sol Defense Group 3/A-5 is a Battle Group of the UNSC. The only information for this group comes from the Halo: Arms Race promotional Halo 3 video released on June 10th, 2007. Very little can be taken from this video and used as hard information, but there are some things that can be assumed.

The group likely was engaged in combat with the Covenant in the First and/or Second Battle of Earth, as shown by the text in the video saying it has 2 ships out of 8. The group requests rearmament since it's readiness level is critical. Factories from the Misriah armory on Mars then are shown producing equipment for the Battle Group, which likely not only uses it ships for space combat, but also assists Marines in ground engagements since the production of Marine vehicles (Warthogs and Pelicans) and weapons (Sniper Rifles,Battle Rifles, and M6D Magnum Sidearms) are shown.

Specifically, the Marine unit (of unknown size) called S4/BAG/2/5 could be assigned to the Battle Group, as it is seen in the video. The Marines themselves are seen in the trailer preparing for combat, and at least one (Benjamin M. Nunez) is not a recruit, while the other Marine(s) pictured likely are.

Speculation
The term Sol Defense Group 3 is likely a large group, or fleet, of the UNSC, and A-5 is possibly a specific designation for the Battle Group.

=Sources=
 * Halo: Arms Race