The route extends for 245.88 miles (395.71 km) between its western terminus at an intersection with NY 104A in the Cayuga County town of Sterling and its eastern terminus at a junction with U.S. Route 9 (US 9) in the Clinton County city of Plattsburgh.

NY 3 traverses eight counties and is a lakeside roadway from Mexico to Sackets Harbor, a mountainous route in Adirondack Park, and an urban arterial in Fulton, Watertown, and Plattsburgh.

In 1924, the segment of the Theodore Roosevelt International Highway within New York was designated NY 3.

At that time, it spanned the full east–west length of the state, extending from the eastern bank of the Niagara River in North Tonawanda to the western edge of Lake Champlain in Plattsburgh; however, the routing through the North Country was significantly different at that time from its modern alignment.

The route was moved onto its modern routing east of Watertown as part of the 1930 renumbering of state highways in New York; it was truncated to Sterling on its western end and rerouted to follow its current alignment from Sterling to Watertown roughly five years later.

Much of this section of the highway is named as part of the Olympic Trail Scenic Byway.

[3] The areas NY 3 passes through alternate between long stretches of rural area, consisting of a mix of open terrain and dispersed residences, and compact settlements containing concentrations of houses and businesses.

Exceptions are in the cities of Watertown and Plattsburgh, where at least part of the road is city-maintained.

From there, NY 3 progresses across Oswego County and passes Lake Neatahwanta prior to entering Fulton.

East of the city, NY 3 passes south of the Oswego County Airport as it heads to the northeast through Palermo.

After crossing the Salmon River, NY 3 passes the Sandy Island Beach State Park before entering Jefferson County.

The route continues northward toward Ellisburg where it meets NY 193 at the entrance to Southwick Beach State Park northwest of the community.

The route bypasses both Sackets Harbor and the Sackets Harbor Battlefield State Historic Site to the east before separating from Lake Ontario and proceeding eastward toward Watertown.

NY 3 forms the southern boundary of the base from NY 3A east to just west of the Jefferson-Lewis County line; however, near Natural Bridge, the Fort Drum boundary heads due north before becoming delimited by the Indian River.

In New York, it connected North Tonawanda (near Niagara Falls) in the west to Plattsburgh in the east via Rochester and Watertown.

NY 3 then followed State Street, Main Street, and East Avenue (modern NY 96) through downtown before turning north onto Culver Road in the eastern portion of the city.

The route remained on Culver Road to Empire Boulevard, where it turned east toward Irondequoit.

[11] NY 3 was realigned by 1930 to stay on East Avenue to Winton Road near the eastern edge of the city.

From there, it utilized the current alignment of NY 37 up through Ogdensburg (by way of Main and Ford Streets in the city) before continuing to Waddington on Van Rensselaer Road.

[10][13] A more substantial realignment of NY 3 took place as part of the 1930 renumbering of state highways in New York.

[16] The most significant realignment that occurred at this time was in the North Country, where NY 3 was shifted onto its current alignment between Watertown and Plattsburgh.

[20][23] It was moved another half-mile (0.8 km) to the east in the early 1960s following the completion of the super two bypass carrying US 104 around the eastern edge of the village.

Both transactions were part of larger highway maintenance swaps between the state and the two counties.

[18][30] An extension of the roadway north to Henderson was completed by the following year.

At least four suffixed routes of NY 3 were created as part of the 1930 renumbering of state highways in New York.