State Route 3 is part of Maine's system of numbered state highways, running from Interstate 95 exit 113 in Augusta to an intersection with Route 102 in the village of Somesville in the town of Mount Desert, on Mount Desert Island. Route 3 is 119.3 miles (192.0 km) in length.

Route 3 uses the Augusta Bypass, a two-lane controlled-access road completed in 2004 to alleviate congestion in the city center (Previous to this, Route 3 followed U.S. Route 202 into Augusta). The bypass ends at U.S. Route 202 on the east end of Augusta, and Route 3 runs concurrently with U.S. 202 (the Belfast Road) to the village of South China, where U.S. 202 leaves to the north. Route 3 continues east to the city of Belfast, where it intersects U.S. 1.

U.S. 1 and Route 3 run concurrently from Belfast through Bucksport, crossing the Penobscot Narrows Bridge over the Penobscot River. From Bucksport, Route 3 and U.S. 1 continue along the Acadia Highway to the city of Ellsworth.

Route 3 leaves U.S. 1 at The Triangle, an intersection at the south end of Ellsworth that has been the center of numerous traffic problems within that city . Continuing south through Trenton, the route crosses a causeway to Mount Desert Island. Route 3 follows the island's eastern shore to the tourist town of Bar Harbor, then continues south through Acadia National Park to the village of Northeast Harbor. The route then turns north, running along with Route 198 to its end near the village of Somesville. Route 3 signs on this last portion of the Route lack directional indicators, as it almost forms a complete loop around the Eastern portion of Mount Desert Island. Signs state "Route" in replacement of directional signs.