Original 1918 route

The original IL 3 route went from Chester in southern Illinois to Morrison in northwest Illinois via Rock Island. With the completion of highway bridges over the Mississippi River (e.g. Clark Bridge at Alton) U.S. Route 67 (US 67) was extended from St. Louis to Godfrey and replaced the original IL 3 to Rock Island. Today, this route is still the major north–south corridor for western Illinois—and the only major Illinois north–south route never converted to the Interstate Highway System like I-57, I-55, or I-39.

Current route

On August 4, 1976, the new Berm Highway from Wood River to Alton was opened.^[3] It was signed as IL 3, which left the old alignment on Lewis and Clark Boulevard and Broadway unmarked. However, on June 29, 1987, the Illinois Department of Transportation (IDOT) built the new Madison Avenue extension in Wood River and marked that road and the Berm Highway as IL 143^[4] and truncated IL 3 at IL 143 (highway signage and IDOT planning maps suggested otherwise, however). This new terminus for Route 3 was short-lived, however.

On November 26, 1987, a new section of Homer M. Adams Parkway in Alton opened to traffic, and IL 3 was extended onto Lewis and Clark Boulevard (a former IL 3 alignment) back into Alton and onto the extension. IL 3 was then cosigned with IL 111 until the intersection with Godfrey Road, where IL 3 takes over the former IL 100 alignment.^[5]