It is the longest state highway in Iowa, at 323 miles (520 km) long.

Iowa 3 begins at the South Dakota state line where it continues as South Dakota Highway 50 and ends at the Northwest Arterial at the Dubuque city limits.

Iowa 3 runs for 323.129 miles (520.026 km) across the northern third of the state, roughly midway between US 18 to the north and US 20 to the south.

All of Iowa 3 east of Le Mars has been listed on the National Highway System,[2] a network of roads important to the country's economy, defense, and mobility.

At Cherokee, the highway intersects US 59 and skirts the northeastern edge of town.

Through Waverly, Iowa 3 is overlapped by US 218 Business before that route turns south to rejoin US 218.

East of Edgewood, Iowa 3 enters the Driftless Area, where the terrain changes markedly from gently rolling farmland to steep bluffs; the intersection with Iowa 38 is located on the edge of the Driftless Area.