BERLIN—A three-way German government led by the Social Democrats (SPD) should be ready to take office by the end of the year, the co-leader of the center-left party said in a newspaper interview published on Saturday. Norbert Walter-Borjans said he was confident his party, which won Sunday’s national election by a narrow margin that left it seeking partners, could move swiftly to agree a coalition with the Greens and the business-friendly Free Democrats (FDP). An SPD ruling alliance with the Greens and FDP—dubbed a “traffic light” coalition after their respective colors—would command a parliamentary majority. “The government should be in place by the end of the year. That is feasible,” Walter-Borjans told the Welt am Sonntag. “We don’t have to hold exploratory talks until we drop this time, because we want a traffic light (coalition) in which all three partners bring their strengths to bear,” he said. “In that sense, …