William Ralph Emerson designed Three Pines just prior to his more famous work in the Shingle Style, using simple stickwork as the dominant motif in the same way he later used wood shingles. For example, the eaves of the roof lack any molded trim that traditionally forms a cornice molding on a building. Instead, the roof serves as a simple covering with wide eaves extending down to encompass the entrance portico and its balcony. This manipulation of structural forms anticipates the all-encompassing roofs of his Shingle Style cottages, as does the saltbox roof on the ell. Typical of his best work, Emerson kept ornamental detail to a minimum.
The house was built for two unmarried sisters, Margaret and Frances Forbes, whose brother was an uncle by marriage of Emerson's wife. This Forbes family connection provided the architect with a number of important commissions in Milton. The design for the house was published in the Boston Architectural Sketchbook in 1876. It was meant to be seen from Brook Road; later development between the house and Brook Road necessitated a new address on Fairfax Road. Now subdivided into apartments, Three Pines remains largely intact on the exterior.