The explosion happened about 9:30 pm on Tuesday as the specialists were trying to make the 500-kilogram US bomb safe after builders found it on a work site where a new stadium is being constructed.
Göttingen police chief Robert Kruse said on Wednesday it was still unclear why the bomb had exploded. The officers killed were aged 38, 52 and 55, he said.
They each had 20 to 30 years' experience and had taken part in defusing about 700 bombs between them, he said, adding it was ''a very sad day.''
Six others were hurt. Two of them, aged 46 and 49, had serious, though not life-threatening, injuries, according to Göttingen city spokesman Detlef Johannson.
All the victims were involved in the defusing operation and were members of the Lower Saxony bomb squad, Johannson said.
Fire department spokesman Frank Gloth told daily Bild: “We'd got the evacuation measures well under way. The workers from the bomb squad began to carry out the first preparatory operations … for the disarming. That's when the bomb exploded.
“In total, 13 workers with the bomb squad were involved in the operation. We are all shocked here. The defusing of a similar bomb a few days ago went smoothly.”
More than 60 years after the end of World War II, weapons recovery remains an important task for police throughout Germany. Allied forces dropped more than 2.7 million tonnes of explosives across Germany during the war. Some of the ordnance did not explode and has become increasingly dangerous with time and corrosion.
Entire neighbourhoods are frequently evacuated for bomb removal, and most are safely defused. Construction and road workers are trained to call emergency services the moment they suspect they've found unexploded ordnance, but accidents still occasionally happen.
In 1994, three construction workers were killed and eight bystanders injured when an unexpected bomb detonated, tearing through nearby buildings and cars in Berlin. In 2006, a road worker was killed near Frankfurt when his excavator hit a bomb.