Infinity Sub-Vessel 3
From Halopedia, the Halo wiki
Later in the campaign, Spartan Commander Sarah Palmer sent Infinity Sub-Vessel 3 to assist Fireteam Crimson at the location known as "Two Giants" when they became pinned down by a CRS-class light cruiser. As Crimson fought off the cruiser's forces with the help of Mantises, Infinity Sub-Vessel 3 destroyed the unsuspecting cruiser with a shot from its MAC gun, allowing Crimson to be safely extracted from the area.
Later, as Infinity was stranded and ambushed at the world of Aktis IV by Jul 'Mdama with the aid of Catherine Halsey, the flagship deployed its entire fleet of Strident-class heavy frigates to ward off the attacking Covenant fleet, which they eventually succeeded in doing.