Towns and cities that Route 3A traverse along its path include Plymouth, Kingston, Duxbury, Marshfield, Scituate, Cohasset, Hingham, Weymouth and Quincy.

This segment parallels Route 3 from Cedarville in southern Plymouth to Neponset in the Dorchester area of Boston.

Towns and cities that Route 3A traverse along its path include Plymouth, Kingston, Duxbury, Marshfield, Scituate, Cohasset, Hingham, Weymouth and Quincy.

The Claire Saltonstall Bikeway between Boston and Cape Cod twice intersects Route 3A.

The section of old Route 3 from Cedarville south to the Sagamore Rotary near the Cape Cod Canal in Bourne is unnumbered.