Route 3A is a 97-mile-long (156 km) state highway in eastern Massachusetts, which parallels Route 3 and U.S. Route 3 from Cedarville in southern Plymouth to Tyngsborough at the New Hampshire state line.

Route 3 is a state-numbered route in the U.S. state of Massachusetts, maintained by the Massachusetts Department of Transportation (MassDOT). Spanning approximately 56 miles (90 km) along a north–south axis, it is inventoried with U.S. Route 3 as a single route by the state. The state-numbered Route 3 travels from Bourne in the south to Cambridge in the north, while US 3 continues from Cambridge and crosses the New Hampshire state line in Tyngsborough. Mileposts on US 3 continue from those on the state-numbered Route 3.