The route begins in Hudson at the Massachusetts line and shortly intersects the eastern end of the Nashua Circumferential Highway, which provides access to US 3 and the F.E.

NH 3A continues along the eastern bank of the river, through Litchfield and into the city of Manchester, where it crosses under I-293/NH 101 and follows a short surface alignment in the southwest section of town.

Hackett Hill Road provides another access point to I-93, while nearby Main Street connects to US 3/NH 28 across the river.

NH 3A enters Bow and meets the southern end of the I-89 freeway before crossing into the city limits of Concord.

At Bristol, 3A leaves the Pemigewasset, which turns east here, and continues north into Bridgewater and the southern shores of Newfound Lake.