The 3rd United States Infantry was also known as Old Guard. During the battle of Gettysburg, it served as a member of Day’s Brigade in Ayres’ Division of the Fifth Corps, Army of the Potomac. The unit was commanded by Henry W. Freedley (d. 1889) of Norristown, Pennsylvania. The infantry unit has 308 men engaged at Gettysburg and suffered 6 casualties with 66 wounded and had a man who went missing.
The 3rd U.S. Infantry - US Regulars Tablet is located on the western most part of Ayres Avenue, at the northern part of a loop which wraps around and takes the road from an easterly direction to a westerly direction. The monument is on the left or west side of the road if traveling south. The monolith is in a clearing, just outside of Rose Woods and has a spectacular view of Little Round Top which is .34 miles away to the southeast. There is a decent-sized shoulder on the right or west side of the road so parking is not an issue here. As there are about seven of these Regulars monuments, pulling over makes sense. Be sure to keep vehicles off the grass or you will be ticketed by park police. I visited this monument on Thursday, July 5, 2012 at 1:38 P.M. I was at an elevation of 570 feet, ASL. I used a Canon PowerShot 14.1 Megapixel, SX210 IS digital camera for the photos.
The monument work was done under the direction of the Gettysburg National Park Commission (established by the United States Department of War), after they took over the administration of the park from the Gettysburg Battlefield Memorial Association (whose funds had expired) on March 3, 1893, and whose stewardship was then transferred to the National Park Service in 1933). SOURCE
Draw the Sword, with descriptive help from the NPS site, offers the following description: One of 45 monuments erected to units of the United States regular army on the battlefield. A red polished Jonesboro granite monolith that is set upon a concrete foundation with a descriptive 3’6'x3’7' bronze tablet with the coat of arms of the United States in bronze. The inscription on this tablet reads:
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Army of the Potomac
Fifth Corps
Second Division First Brigade
Third U.S. Infantry
Six Companies
Captain Henry W. Freedly and Captain Richard G. 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 retired under a deadly fire on the left right and 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 same position
July 4 The Regiment with the Brigade made a reconnoissance and developed a force of the Confederate Infantry and Artillery in force.
Casualties killed 3 men wounded 4 officers and 83 men missing 1 man.
There are hundreds of non-sculpted monumentation for the various Confederate States & Union units which fought at Gettysburg. There are about 10 different variations of tablets, markers & monoliths by my reckoning. Six designs represent brigade (2), division (2), corps (2), & army headquarters (2), each different in a subtle way so as to distinguish between armies. Battalion/battery/advance position markers & regimental monuments for U.S. Regulars & Confederate have their own distinct design (2), bringing the total to 10 designs at the battlefield. The bronze tablets were created by Albert Russell & Sons Co. of Newburyport, Mass. The granite bases which accompany the various tablets were created by the Van Amringe Company from Boston, Mass. The cast iron tablets were manufactured by Calvin Gilbert. The 1910 Gettysburg Commission report lists the awarded contracts to these companies (not Gilbert) for the tablets. All of these tablets were designed by architect Colonel Emmor Bradley Cope (July 23, 1834 - May 28, 1927). He designed pretty much every tablet for both the Union & Rebel armies, each one distinct, w/ several different varieties. He is buried in Evergreen Cemetery 
in Gettysburg, as well he should be as this park and most of the things seen here today were designed by him and as such, are his legacy. These monuments were erected just after the turn of the century during the first & beginning of the second decade of the 20th century. Each one has since been preserved or restored at least twice since the turn of the 21st century. The plaques & tablets were erected by the Gettysburg Park Commission (established by the War Dept.). The Gettysburg Park Commission is also referred to as the Gettysburg National Military Park Commission or the Gettysburg National Park Commission, to clear up any confusions, especially my own.
On October 1, 1898, the Gettysburg National Park Commission in a letter to the Secretary of War set gave recommendations for continuing the task of organizing & progressing the work of the Gettysburg National Military Park. Every November they wrote their annual report which outlined the work of the GNPC for that year. The following is an excerpt from that report relevant to this waymark. The link at the end of the paragraph will take you to the entire report. The method of marking the positions of troops on this field, as approved by the War Department, is to place the principal tablet or monument of each command at the position occupied by the command in the main line of battle, and to mark the several important positions subsequently reached by each command in the course of the battle by subordinate and ancillary tablets, with appropriate brief inscriptions giving interesting details and occurrences and noting the day and hour as nearly as possible. SOURCE
Most of the Commission reports have been digitized and can be found HERE. The initial 1893 report up to & including 1921 can be found here. 1922 to 1926 are missing & have yet to be discovered. The years 1927 to 1933 were discovered in 1996 and are also included. To my knowledge this is the most comprehensive list of documents that illustrated the development and formation of the park.
Information about these specific types of monuments:
UNITED STATES REGULARS TABLETS
Forty-two monuments honor units of the regular army to fight at Gettysburg. Other sources have the amount at forty-four tablets while I have personally counted 48 of these monuments. These regiments and batteries were designated as United States Regulars as opposed to the state volunteer forces. These polished granite monuments with bronze plaques affixed to the front were fabricated by the Van Amringe Granite Company, of Boston and authorized in 1907. They were completed in the autumn of 1908. Each consists of Jonesboro granite, 24 by 50 inches and 7 feet high, set upon concrete foundations, and upon each is fastened a descriptive bronze tablet and the coat of arms of the United States. are similar in appearance to and often mistaken for headquarters, since they have the same curved-and-scrolled top on the bronze tablet as corps and army headquarters. The difference is that instead of being square, the stone comes to a peak in the center on the U.S. Regulars monuments, with a brass Great Seal of the United States centered within the peak. SOURCE & SOURCE
The 3rd U.S. Infantry - US Regulars Tablet is a contributing feature to the Gettysburg Nat'l Military Park H.D. which is nationally significant under NR Criteria A, B, C & D. Areas of Significance: Military, Politics/Gov't, Landscape Architecture, Conservation, Archeology-Historic. Period of Significance: 1863-1938. The monument is designated as structure no. MN646.
From the Nomination Form:
1 of 45 Civil War US Regular Tablets in Park. Locates positions occupied by Regular 5th Infantry during Battle of Gettysburg, July 2, 1863. Located on E side of Ayres Ave.
Short Physical Description:
Red granite monolith w/rough-hewn sides & back, 4'2"x2', 7" high. Gable top. Bronze inscription tablet, 3'6"x3'7", mounted on polished face. Bronze seal of US Coat of Arms, 1' in diameter, on face above tablet.
Long Physical Description:
N/A
My Sources
1. NRHP Nomination Form
2. Stone Sentinels
3. Virtual Gettysburg
4. Draw the Sword
5. Historical Marker Database