From the monument:
Army of the Potomac
Second Division, First Brigade
Third U.S. Infantry
Captain Henry W. Freedly and Captain Richard C. Lay commanding
July 2 Arrived in the morning and took position near the line of the Twelfth Corps. The Regiment with the Brigade moved from the right to the left of the line and at 5 p.m. advanced across Plum Run near Little Round Top and supported the Second Brigade in its advance to the crest of the rocky wooded hill beyond and facing to the left. Engaged the Confederates but returned under a deadly fire on the left right and rear rear after the Confederates had gained a position in the Wheatfield in the rear of the Brigade and took position on east slope of Little Round Top.
July 3 Remained in the same position
July 4 The Regiment with the Brigade made a reconnaissance and developed a force of the Confederate Infantry and Artillery in front.
Casualties: killed 8 men, wounded 4 officers and 63 men, missing 1 man.
See more on the 3rd United States Infantry Regiment in the Civil War