Both routes, which connect end-to-end, are treated as a single 91.3-mile (146.9 km) state highway by the Massachusetts Department of Transportation (MassDOT).

From Cambridge to Burlington, US 3 is routed on surface streets through the dense suburbs in the Greater Boston area.

From where it leaves the Everett Turnpike in Nashua northward, US 3 is generally a two-to-four lane at-grade road, though there are two super-two freeway portions in northern New Hampshire, one on the Laconia Bypass, and one where US 3 and I-93 use the Franconia Notch Parkway.

The route serves as a major local arterial, connecting many of the cities of the densely populated Merrimack Valley.

US 3 begins in the south along Memorial Drive in Cambridge, along the Charles River, at an interchange with Massachusetts Avenue (Route 2A).

It runs along the north bank of the Charles River, opposite Soldiers Field Road in Allston along this stretch.

US 3 continues through parts of Winchester and Woburn without any major intersections before entering Burlington and interchanging with I-95 and Route 128 (Yankee Division Highway) at exit 51A.

US 3 joins the freeway to connect with the Northwest Expressway, while its historic surface alignment continues as Route 3A.

The freeway closely parallels Route 3A, the historic alignment of US 3, along its entire 19-mile (31 km) length from Burlington to the New Hampshire state border.

US 3 crosses the state border into Nashua and immediately becomes concurrent with the Everett Turnpike, running on the freeway for 6.7 miles (10.8 km) along the western side of the city.

The highway parallels I-293 until it turns east in Manchester and then crosses the Merrimack River on Queen City Avenue, just after its intersection with I-293/NH 3A and NH 114A.

US 3 and NH 3A are signed in a wrong-way concurrency for approximately 0.6 miles (0.97 km) before US 3 turns north onto Elm Street toward downtown Manchester.

After approximately 2.2 miles (3.5 km), US 3 turns east onto Webster Street, then joins NH 28 to proceed in a northeasterly direction toward Hooksett, interchanging with I-93.

After crossing the Merrimack River and interchanging with I-93, US 3 intersects NH 3A (South Main Street), which terminates at its parent route.

Continuing north, US 3 joins with I-93 as it passes through Franconia Notch State Park, one of the more scenic drives in the White Mountains.

This stretch of freeway is known as the Franconia Notch Parkway and is a rare section of Interstate Highway with only one lane in each direction.

Heading north from Twin Mountain, US 3 passes through the village of Carroll, where NH 115 branches to the northeast and US 3 bears to the northwest and the town of Whitefield.

After US 2 leaves to the west, US 3 continues north, roughly paralleling the course of the Connecticut River (which also forms the border with Vermont), through Northumberland and Groveton, where NH 110 ends.

Sections of US 3 in Colebrook are named after Scott E. Phillips and Leslie G. Lord, members of the New Hampshire State Police killed in the line of duty on August 19, 1997.

The roadbed and bridges were built to support a fourth lane in each travel direction for future expansion.