BERLIN—Germany’s Olaf Scholz is on course to succeed Angela Merkel as chancellor after the victors of the September election reached a policy agreement to form the country’s first three-party coalition, people involved in the negotiations said.

The policy program, which Mr. Scholz’s center-left Social Democrats, the environmentalist Greens and the pro-business Free Democratic Party are set to unveil on Wednesday afternoon, aims to overhaul the German economy with a focus on combating climate change and updating the country’s patchy digital infrastructure.