It’s usually claimed that Millay was inspired by the view from Mount Battie near Camden, Maine, where she lived as a child. There’s even a plaque attached to a rock near the summit:
Photo by Roger W. Sinnott (2010), via the Historical Marker Database.
The claim seems plausible for biographical reasons. ‘Renascence’ was written in 1912, and until she went to Vassar College in 1913, Millay lived in Camden, from whence the summit of Mount Battie is about a mile’s walk:
She spent a great deal of her time out of doors, sometimes on solitary excursions up Mt. Battie or Mt. Megunticook, sometimes on walks or picnics with her sisters or a group of friends. The people of Camden used the mountains constantly as part of the terrain of their daily lives. Bird walks in the early, often pre-dawn, mornings were a favorite summer diversion. They would take breakfast along, stopping to eat on a mountain ledge lit by the first rays of the sun, with the ocean spread out beneath them. In the spring Vincent would lead her friends to Mt. Battie, where her own ramblings had disclosed fields of mayflowers. In autumn, when the whole countryside blazed into flame and gold, the girls walked and climbed almost without end.
Miriam Gurko (1962). Restless Spirit: the Life of Edna St. Vincent Millay, p. 30. New York: T. Y. Crowell.
From the summit of Mount Battie, there are views to other peaks, including Mount Megunticook, Bald Mountain, and Ragged Mountain; and views over Penobscot Bay, which contains many islands. It’s doubtful that Millay intended a specific point from which one can see exactly three mountains and exactly three islands: but rather, for poetic effect she made the mountains and islands parallel in number.
Courtesy of Google Street View, here are three views from the summit of Mount Battie. The first two are from the Memorial Tower, built in 1921 to recognize the casualties of the World War, so post-dating ‘Renascence’.
(i) Looking west across the valley of the Megunticook River (winter).
(ii) Looking north to Mount Megunticook (winter).
(iii) Looking east across Penobscot Bay (summer).