King's Highway 3, commonly referred to as Highway 3, is a provincially maintained highway in the Canadian province of Ontario which travels parallel to the northern shoreline of Lake Erie. It has three segments, the first of which travels from the Ambassador Bridge in Windsor to Highway 77 in Leamington. The second portion begins at Talbotville Royal outside of St. Thomas at Highway 4, and travels to the western city limits of Port Colborne. The road is regionally maintained within Port Colborne as Niagara Regional Road 3, but regains its provincial designation at Highway 140. Its third and final terminus is at Edgewood Park, within the Fort Erie town limits. From there, the road continues as Niagara Regional Road 3 to the Peace Bridge, where drivers can cross to the United States. The total length of Highway 3 is 258.2 km (160.4 mi), consisting of 49.2 km (30.6 mi) from Windsor to Leamington, 187.9 km (116.8 mi) from Talbotville Royal to Port Colborne and 21.1 km (13.1 mi) from Port Colborne to Edgewood Park.

The Ambassador Bridge is an international suspension bridge across the Detroit River that connects Detroit, Michigan, United States, with Windsor, Ontario, Canada. Opened in 1929, the toll bridge it is the busiest international border crossing in North America in terms of trade volume, carrying more than 25% of all merchandise trade between the United States and Canada by value. A 2004 Border Transportation Partnership study showed that 150,000 jobs in the Detroit–Windsor region and US$13 billion in annual production depend on the Detroit–Windsor international border crossing.