The 3rd Maine Regiment was mustered in at Augusta, Maine, for three years' service on June 4, 1861, and were mustered out on June 28, 1864. Veterans who had re-enlisted and those recruits still liable to serve were transferred to 17th Maine Volunteer Infantry Regiment. The regiment included Company A (Bath City Grays).

Of the 1,586 men enrolled in the regiment, 134 were killed or died of wounds, 140 died of disease, and 33 were in Confederate prisons.

They served in the following engagements:

• First Manassas (First Bull Run, VA) - July 21, 1861
• Bailey's Crossroads, VA - August 28, 1861
• Yorktown, VA - April 5 - May 4, 1862
• Williamsburg, VA - May 4, 1862
• Seven Pines, VA - May 31, 1862
• White Oak Swamp, VA - June 30, 1862
• Malvern Hill, VA - July 1, 1862
• Second Manassas (Second Bull Run, VA) - August 29-30, 1862
• Chantilly, VA - September 1, 1862
• Fredericksburg, VA - December 13, 1862
• Chancellorsville, VA - May 1-4, 1863
• Gettysburg, PA - July 1-3, 1863
• Wapping Heights, VA - July 21-23, 1863
• Catlett's Station, VA - October 14, 1863
• Kelly's Ford, VA - November 7, 1863
• Orange Grove, VA - November 26, 1863
• Mine Run, VA - November 30, 1863
• Wilderness, VA - May 5-7, 1864
• Spotsylvania, VA - May 7-20, 1864
• North Anna, VA - May 23-27, 1864
• Totopotomy, VA - May 26-30, 1864
• Cold Harbor, VA - May 31-June 12, 1864