The three suspects, all in their 90s, were members of the so-called "Hitler Youth" SS division blamed for killing 86 people in the northern French town of Ascq.
The head of the central Nazi war crimes investigation unit in the western state of North Rhine-Westphalia, Andreas Brendel, told AFP that the probe also covered other unspecified war crimes between April 1944 and May 1945.
"We also searched the homes of other people and seized documents linked to the Hitler Youth division and the probe into the Ascq massacre, but these people are for the moment only considered witnesses" and not suspects, Brendel said.
On the night of April 1st, 1944, 86 people were shot dead in Ascq in revenge for an attack on a troop transport of the "Hitler Youth" division.
"The accused denied during questioning having been members of the division and thus any participation in the killings," following the raids carried out on Tuesday and Thursday, Brendel said.
He said the confiscated documents were now being scoured for clues. Should the investigation lead to criminal charges, the ability of the aged suspects to stand trial must also be assessed.
"This is always a serious problem with elderly suspects," he said.
"Our aim is to shed light on what happened - it is above all about establishing legal clarity on the Ascq massacre."
Brendel also led the investigation into a German former soldier over the Nazis' worst atrocity on French soil, the 1944 slaughter of 642 people in the tiny village of Oradour-sur-Glane.
A court in the western city of Cologne finally threw out the case in 2014 against Werner Christukat, then 89, citing a lack of evidence.
The new investigations are part of a twilight bid by the German justice system to prosecute crimes committed by the Nazis before the last perpetrators die.
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Around a dozen investigations are currently under way against former SS officers who worked at Nazi concentration camps.
The so-called "Bookkeeper of Auschwitz" Oskar Gröning was sentenced to four years in jail last year as an accessory to murder in 300,000 cases in which Hungarian Jews were sent to the gas chambers between May and July 1944.
Another two cases involving Auschwitz personnel are due to go on trial next month.