Watertown to Plattsburgh
In Black River, northeast of Watertown, NY 3 intersects NY 342 at the southwestern tip of the Fort Drum Military Reservation. NY 3 follows the southern edge of the base to Deferiet, where NY 3 turns south to follow the Black River while NY 3A continues along the border of Fort Drum. NY 3 follows the river to Carthage, where it overlaps NY 126 briefly before heading northeast to rejoin NY 3A at Fargo. 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. NY 3, meanwhile, traverses the river and enters Lewis County.
NY 3 eastbound at Franklin CR 26 (former NY 99) in the town of Franklin
Midway between Natural Bridge and Harrisville, NY 3 intersects NY 812. NY 812 turns east onto NY 3, following NY 3 northeast into St. Lawrence County before splitting to the northwest between Harrisville and Pitcairn. Shortly after departing NY 812, NY 3 enters Adirondack Park. The route follows a largely east–west routing from its entry point to Tupper Lake, where it merges with NY 30. Near Upper Saranac Lake, NY 30 heads north toward Malone as NY 3 continues northeast through nearby Saranac Lake toward Plattsburgh.
West of Plattsburgh, NY 3 exits the park and encounters NY 374 via an interchange. Past the exit, the two routes follow parallel routings before separating outside of Plattsburgh. NY 3 curves to the southeast toward the former Clinton County Airport, where it meets NY 22B just west of what was once the main entrance to the airport. From NY 22B, NY 3 continues east to the outskirts of Plattsburgh, where it meets the Adirondack Northway (I-87). Past I-87, NY 3 follows Cornelia Street into the city, where it becomes city-maintained. Within Plattsburgh, NY 3 intersects NY 22 before terminating at US 9 just west of Lake Champlain.