The 3rd Massachusetts Artillery Battery C was also known as Martin’s Battery. During the battle of Gettysburg, it served as a member of Martin’s Brigade in the Fifth Corps, Army of the Potomac. The battery was commanded at the Battle of Gettysburg by Lieutenant Aaron Francis Walcott (1836 - December 11, 1907), previously, a bookkeeper from Boston. Walcott enlisted April 25, 1861 and mustered in on July 31, 1861. He mustered out on September 16, 1864 but then re-enlisted and drilled the battery in Boston. In 1869 and 1870 he was a merchant tailor, then he went into the real estate business, staying in Boston. IN 1872, he moved to Chicago, continuing real estate an also the storage business (forward thinker?). In 1900 he returned to Massachussets and lived in Melrose. While in Chicago he served as a deputy sheriff of Cook County for 8 years. Under Walcott's command the unit brought 124 men to the Gettysburg Battlefield operating 6 Napoleons. There was a total of 6 casualties.
The monument to the Third Massachusetts Battery is south of Gettysburg on Wheatfield Road. More specifically, the monument is located near the Weikert Farm Lane. Located at the corner of the F.Althoff/John Weikert Lane and Wheatfield Road in the Plum Run Valley. There are several monuments on the right side of this road in front of the house in this open stretch of green. Two 12-pounder Napoleons represent the battery at this location and are on either side of the monument. On the left is registry number 177 produced by Revere Copper in 1862. On the right is a "rifled" version of the gun produced by Ames Manufacturing in 1862, bearing registry number 77. 
Parking is available on the side of the road in intermittently place shoulders. Be sure to stay off of the grass or the Park Service will issue you a ticket.
The Draw the Sword site helped out by the NPS narrative and the SIRIS site offers the following description: Monument: granite; Base: stone. A granite monument with finial of five polished gray granite cannon balls. The front of the monument has a polished V Corps insignia design, with artillery insignia emblackened. (The front of the monument features an artillery badge supporting the Maltese cross of Fifth Corps.) Monument is a 1.10 foot square granite shaft topped with four polished granite balls and set on a 3.6 foot base. Overall height is 6.8 foot. The shaft is inscribed on all faces with a polished insignia of the V Corps and artillery and raised letters on the south face.
Dedicated October 8, 1885, it marks the position held by the 3rd Massachusetts unit on the late afternoon of July 2, 1863. The monument was fabricated by the Smith Granite Company and is composed entirely of granite. SIRIS lists the dimensions as Overall: approx. 6 ft. 8 in. x 3 ft. 6 in. x 3 ft. 6 in. This is one of 25 monuments in the park honoring Massachusetts troops in the Gettysburg campaign. There are two very brief inscriptions on the front and reverse sides which read.
(Front):3rd Mass. Battery
5th Corps Art'y.
July 2, 1863.
The 3rd Massachusetts Battery Monument, also known as the Walcott's Battery Monument, is a contributing feature to the Gettysburg National Military Park Historic District which is nationally significant under NR Criteria A, B, C & D. Areas of Significance: Military, Politics/Government, Landscape Architecture, Conservation, Archeology-Historic. Period of Significance: 1863-1938. The original National Register Nomination was approved by the Keeper March 19, 1975. An update to this nomination was approved by the Keeper on January 23, 2004. The monument is identified as structure number MN117-B.
From the Nomination Form:
Marks position held by 3rd Massachusetts Battery on July 2, 1863. 1 of 25 mns in Park to Massachusetts commands. Located at corner of F. Althoff/John Weikert Lane and Wheatfield Rd. near Plum Run Valley.
Short Physical Description:
Mn & 2 cannons. Base 3'6" sq. Granite shaft, 1'10" sq. topped w/ 4 polished granite balls. All 6'8" high. Shaft inscribed on all faces, polished insignia of V Corps & artillery on S face. Raised letters on S face. Flanked by 2 bronze Napoleon cannons on cast-iron carriages.
Long Physical Description:
Monument that has two flanking bronze cannons. Monument is a 1.10 foot square granite shaft topped with four polished granite balls and set on a 3.6 foot base. Overall height is 6.8 foot. The shaft is inscribed on all faces with a polished insignia of the V Corps and artillery and raised letters on the south face. Two Napoleon cannon on cast iron carriages flank the monument. Located at the corner of the F.Althoff/John Weikert Lane and Wheatfield Road in the Plum Run Valley.
My Sources
1.
NRHP Narrative
2.
SIRIS
3.
Stone Sentinels
4.
Virtual Gettysburg
5.
Draw the Sword
6.
Historical Marker Database