John Custis III was the father of John Custis IV and son of John Custis II, the immigrant. 19th-century descendants remember a number of Custis family portraits hanging at Abingdon. While it cannot be confirmed that this subject was painted, it seems highly likely that his was among the lost portraits at Abingdon in the 19th century. Many of his Custis relatives were painted. Since he was a captain and then colonel in the militia, he may have been the subject wearing a suit of armor.
Possible reference: “There were some portraits of the Custis family at Abington, on the Potomac, which have long since crumbled into dust. One who bore the name of Custis is remembered as being represented as a soldier in a complete suit of armor,” in Benson J. Lossing, Recollections and Private Memoirs of Washington (1860), 21.