Beginning in January 1863, and continuing until early 1864, twelve companies of heavy artillery were raised in Massachusetts and mustered into service to garrison the military forts along the coast of the state. The units were designated "unattached" as they did not belong to a particular regiment.
In the spring of 1864, the 3rd Unattached Company, along with the 6th through 12th, were ordered to Washington, DC, where they were to garrison the forts protecting the capital. In June of 1864, the 6th, 7th, 8th, 9th, 10th, 11th, 12th, and 14th independent companies were part of the 3rd brigade, within Joseph A. Haskin's division. At the same time, the 3rd and 15th companies were part of the 2nd brigade within Gustavus De Russy's division.
Massachusetts' Governor Andrew called for the companies to be given regimental status, and when it was granted, several other companies since raised were sent to Washington to complete its complement, and was officially made a regiment in the fall of 1864. Except for Company I, the regiment continued with its duty of manning forts in the vicinity of the capital. A portion of the regiment was mustered out on 17 June 1865, while the remaining companies served until 18 September.
The 3rd Regiment, Massachusetts Heavy Artillery was organized for one year August, 1864, by consolidation of 3rd, 6th, 7th, 8th, 9th, 10th, 11th, 12th, 13th, 14th, 15th and 16th Unattached Companies Heavy Artillery. In April of 1865, most of the regiment was part of the Army of the Potomac commanded by major general George Meade. They were split between the second and third brigades in brigadier general Martin Davis Hardin's division. Mustered out September 18, 1865. The Regiment lost during service 2 Enlisted men killed and 1 Officer and 38 Enlisted men by disease. Total 41.
Attached to 2nd Brigade, Hardin's Division, 22nd Army Corps, Dept. of Washington, and engaged in garrison duty in the Defenses of Washington, north of the Potomac, to September, 1865. (For Co. "I," 13th Unattached Company, see 13th Unattached Company.)
The composite roster of this unit contains the names of 2864 men.
Second Brigade (Colonel William S. Abert)
Company A - Captain Benjamin A. Ball
Company D - Lieutenant Lewis R. Whittaker
Company G - Captain Thomas Herbert
Company H - Captain George W. Pierce
Company K - Captain Edwin Thomas
Company L - Captain Joseph M. Parsons
Company M - Captain Cornelius F. Driscoll
Third Brigade (Major George S. Worcester)
Company B - Lieutenant James E. Childs
Company C - Captain Alfred W. Brigham
Company E - Captain Leonard Gordon
Company F - Captain Joseph Austin
The 13th Unattached Company, which later became Company I, was recruited in Springfield, MA and was largely composed of mechanics who were employed at the city's National Armory. Because of their engineering skills, they were detached from the regiment and attached to the Army of the James, and were engaged in building bridges and roads. During the Siege of Petersburg, they were in charge of holding the pontoon bridge placed across the James River. They were the last men of the regiment to be mustered out, doing so on 26 September 1865.