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.