U.S. Route 3 (US 3) is a north–south United States highway that runs from its southern terminus in Cambridge, Massachusetts, through New Hampshire to its terminus near Third Connecticut Lake at the Canadian border, where the road continues north as Quebec Route 257.

In New Hampshire, parts of US 3 are known as the Daniel Webster Highway. From Burlington, Massachusetts to Nashua, New Hampshire, US 3 is a freeway. (The 6.7 miles (10.8 km) in New Hampshire are a free portion of the Everett Turnpike.) A second freeway portion exists in northern New Hampshire, where US 3 utilizes the Franconia Notch Parkway and a short segment of Interstate 93, before proceeding on two-lane roads north to the border with Canada. While the southern terminus has not changed, there have been different northern termini to this route.

Massachusetts Route 3 continues beyond Boston to Cape Cod, but has never been part of US 3.

In Massachusetts, US 3's freeway portion heading from Interstate 95/Route 128 to the New Hampshire border is numbered using the original exit numbering system from the days when US 3's freeway portion was to extend south of Route 128 to Boston to connect with Route 2 and the never-built Inner Belt (I-695). The numbering "begins" with Exit 25 at Route 128 and proceeds north. All other freeways which once used that system have been renumbered with more conventional exit numbers. Because of the cancellation of Route 3 as an expressway inside of Route 128, one travelling on Route 3 will travel along a wrong-way concurrency with I-95/Route 128 for a short while (going South on Route 3 is I-95/Route 128 North), as to link up the "old" Route 3 inside of the belt and the "new" Route 3 outside of it.

The "old" Route 3 runs south from I-95/Route 128 Exit 33, through portions of Burlington, Woburn, and Winchester, without intersecting any other numbered routes. In Arlington, it joins with Massachusetts Routes 2A and 60 in Arlington Center. The concurrency of Routes 2A, 3, and 60 is a three-way concurrency; for two blocks on Mystic Street, one is traveling south on US 3, east on Route 2A, and west on Route 60. At Massachusetts Avenue, US 3 and Route 2A turn east toward Boston until they reach the Alewife Brook Parkway and Route 16, at which point Route 3 joins those routes heading south. At the Alewife MBTA Station, Routes 3 and 16 join Route 2 and the Fresh Pond Parkway bearing south and east to Memorial Drive in Cambridge. The concurrency of Routes 2A, 3, and 16 is a three-way concurrency; between Alewife Station and Huron Avenue, one is traveling south on US 3, east on Route 2, and west on Route 16. US 3 ends on Memorial Drive at the point where it meets Massachusetts Route 3.

US 3 is one of New Hampshire's most well-known roads, as it passes through most of the state's major cities and towns. It is the only signed/numbered route in the state to extend fully from its southern border with Massachusetts to its northern border with Quebec; thus, it is also the longest route within New Hampshire, at 242 miles (389 km).

Route 3 crosses the New Hampshire-Massachusetts border, leaving the town of Tyngsborough, Massachusetts and entering the city of Nashua, New Hampshire, and becomes known as the Everett Turnpike. Route 3 leaves the Everett Turnpike at Exit 7E in Nashua and turns northeast for approximately 1.5 miles (2.4 km) along a segment known as the Henri Burque Highway, before turning north onto Concord Street, which soon becomes known as the Daniel Webster Highway. Many locals will erroneously refer to the Everett Turnpike from Exit 7 through the I-293 interchange as Route 3 and refer to the actual Route 3 only as the Daniel Webster Highway. Route 3 continues north through the town of Merrimack and into Bedford, where it becomes South River Road. Route 3 parallels Interstate 293 until it turns east, crossing the Merrimack River on Queen City Avenue approximately three-quarters of a mile north of the Bedford-Manchester line. After approximately three-quarters of a mile, Route 3 then turns north onto Elm Street in downtown Manchester. After approximately 2.2 miles (3.5 km) on Elm Street, Route 3 turns east onto Webster Street, then joins NH Route 28 to proceed in a northeasterly direction on another segment of the Daniel Webster Highway.

In Suncook, New Hampshire, Route 28 leaves to the northeast, and Route 3 proceeds northwest towards Concord on Pembroke Street, becoming Manchester Street when it enters the Concord city limits. Route 3 traverses downtown Concord, where it is known as North and South Main Street, then follows North State Street to Fisherville Road to Village Street in Penacook before crossing the Contoocook River into Boscawen. Route 3 travels north through Boscawen, briefly joining US 4. Route 3 parallels the Merrimack River north into Franklin. In Franklin, the route joins New Hampshire Route 11 and turns east, briefly coinciding with New Hampshire Route 127, and passing through Tilton, where it crosses NH 132 and passes the western end of NH 140. Continuing northeast past Winnisquam Lake, Routes 3 and 11 reach Laconia and turn onto the Laconia-Gilford Bypass, passing interchanges with New Hampshire Routes 106, 107, and 11A. At the north end of the bypass, Routes 3 and 11 diverge, with Route 3 continuing north on Lake Street to Weirs Beach and an intersection with 11B. Route 3 continues north as the Daniel Webster Highway to Meredith at the north end of Meredith Bay on Lake Winnipesaukee. In Meredith, after passing the northern terminus of NH 106, Route 3 joins NH 25 and continues north past Squam Lake into Holderness, passing the west ends of NH 25B and NH 113. Through Holderness, Routes 3 and 25 gradually turn west, then southwest, passing the southern end of NH 175, and then reaching the northern end of NH 132 in Ashland.

From Ashland to North Woodstock, Route 3 proceeds north, roughly paralleling I-93 in the Pemigewasset River valley. Along this stretch it passes through the towns of Plymouth (where NH 25 diverges to the west), West Campton (where it meets the western end of NH 49, the principal access road to Waterville Valley), Thornton, and Woodstock. In North Woodstock, Route 3 crosses NH 112 (also known as the Kancamagus Highway to the east). Continuing north, Route 3 joins with I-93 as it passes through Franconia Notch State Park, one of the more scenic drives in the White Mountains. It separates from I-93 shortly north of the northern park boundary in Franconia, about 1.5-mile (2.4 km) north of the exit with NH 18. From there, after NH 141 branches northwest, Route 3 heads north and east toward Twin Mountain and a junction with U.S. Route 302. This portion of the road is noted for fairly frequent moose sightings, especially during sunrise and sunset when moose are particularly active.

Heading north from Twin Mountain, Route 3 passes through the village of Carroll, where NH 115 branches to the northeast, and Route 3 bears to the northwest and the town of Whitefield. In the center of Whitefield, NH 142 branches to the northwest, and NH 116 crosses, running roughly southwest to northeast. Route 3 continues north to Lancaster, where it joins U.S. Route 2 in the town center, and where NH 135 branches left. After US 2 leaves to the west, Route 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. North of Groveton, Route 3 continues to follow the river, through Stratford, North Stratford, and Columbia, until it reaches Colebrook, where New Hampshire Routes 26 and 145 begin. Still following the Connecticut River north, Route 3 passes through portions of Stewartstown and Clarksville. In Stewartstown, the road turns more directly east (still following the Connecticut River, which is no longer a boundary), before resuming a northeasterly direction through Pittsburg, where it meets the northern end of NH 145, eventually heading directly north to the Canadian border crossing at Chartierville, Quebec, where it becomes Quebec Route 257.

Prior to the U.S. Highway system, the section of US 3 from Orleans, Massachusetts to Colebrook, New Hampshire was part of New England Interstate Route 6 (NE-6). It was replaced in its entirety with the establishment of US 3 in 1926.

Route 3 in Massachusetts (from Tyngsborough to Cambridge) closely follows the route of the early 19th Century Middlesex Canal and Middlesex Turnpike.

The modern expressway was begun around Massachusetts Route 110 at Lowell before World War II. In the 1950s, it was extended south to Route 128, and by the 1960s, it was completed north from Chelmsford to New Hampshire. In the first half of the first decade of the 21st century, the chronically congested four-lane road, largely with antiquated ramps around Lowell, was widened to six lanes (as it had been in Nashua, New Hampshire a few years previously), and many interchanges were modernized in what was comically known as "The Big Wide," in reference to Massachusetts' other "Big" construction project (the Big Dig). The $365 million 21-mile (34 km) widening project was completed in 2005 from Burlington to the New Hampshire border.

According to the AASHTO route log, the southern terminus of US 3 is at US 20 in Boston, after crossing the Charles River at the Boston University Bridge. However, MassDOT officially places the southern terminus at the junction of Route 2A and Route 3 in Cambridge, which is where Route 2A crosses the Charles along the Harvard Bridge (also known as the Massachusetts Avenue Bridge). The original northern terminus (in 1926) was Colebrook, New Hampshire, but the highway was extended to West Stewartstown, New Hampshire in 1928, and to Pittsburg, New Hampshire in 1937. Colebrook was temporarily the northern terminus again from 1939 to 1940. Since 1940, the highway has run through Pittsburg to the New Hampshire-Quebec border.

U.S. Route 3 Business is a 4.20-mile (6.76 km) long business route running north–south through downtown Laconia, New Hampshire. It runs from US 3 in Laconia south to US 3 and NH 11 in Belmont, New Hampshire along New Hampshire Route 107 and New Hampshire Route 11A.