It crosses the Longfellow Bridge on the Boston–Cambridge city line, and transitions into US 3 shortly after an intersection with Route 2A in Cambridge.

Massachusetts Route 3 can be said to have three segments: the Pilgrims Highway, the concurrency with Interstate 93, and the section north of I-93 in Boston.

The Pilgrims Highway is the part of Route 3 that stretches from the intersection with US 6 in Bourne to the junction with Interstate 93 in Braintree.

Between the towns of Pembroke and Hingham, the highway is typically composed of four lanes, but is expanded to five during rush hour.

After the interchange with the Massachusetts Turnpike, just before the route enters the Tip O'Neill Tunnel in Downtown Boston, it is reduced to six lanes.

The route was basically a connected system of two-lane roadways up until the 1950s with the exception of Route 3's original path through Boston which paired it with US 1 on Park Drive, the Riverway and the Jamaicaway and then along its own path in Mattapan and Dorchester along the Arborway, Morton Street and Gallivan Boulevard.

The northern section of the highway was built next with a connection from Derby Street in Hingham to the Southeast Expressway opening in 1959.

Route 3 was connected to the Southeast Expressway in Milton by using Granite Avenue as a link from Gallivan Boulevard.

Route 3 was then taken off its remaining pathway along surface streets in Boston and extended up the Southeast Expressway and Central Artery in 1971 to the Storrow Drive exit.

[2] MassDOT is looking into the possibility, in a similar manner to what happened to a 14+ mile section of Route 128 in the 2010s, of widening Route 3 from I-93 to the Sagamore Bridge from four to six lanes (as has already been done within the US 44 road concurrency between exits 15 and 16 near Plymouth) with new bridges and new on- and off-ramps, and will include upgraded interchanges with new acceleration and deceleration lanes.