The investigation into one of them has revealed he had links to a network of people that were involved in a gunman's thwarted August 2015 attack on a Paris-bound train, a source briefed by German officials told CNN on condition of anonymity.
Information gathered so far seems to indicate that the three -- arrested Tuesday morning in the northern state of Schleswig-Holstein by special police forces -- were brought to Europe by the same organization of smugglers that imported the Paris attackers, German Interior Minister Thomas de Maiziere said in Berlin.
Their travel documents also appear to have originated from the same forger, de Maiziere said.
The three -- two Syrian teenagers and one man in his 20s, identified as Mohamed A. -- came to Germany in the middle of November 2015 via Turkey and Greece, the German Federal Prosecutor's Office said.
Tuesday's arrests come as European security officials hunt for people connected to those who killed at least 130 people
in gunfire and suicide bombings at six locations in the Paris area on November 13.
European security officials had estimated that 30 to 40
suspected ISIS terrorists who helped support the November Paris attacks are still at large, CNN reported last week.
De Maiziere did not say what connection Tuesday's three arrested suspects may have had with the Paris attacks, other than coming into Europe via the same smuggling group that brought in the Paris attackers.
Three allegedly came to Germany on ISIS' orders
Investigators suspect the three of coming to Germany on the orders of ISIS in November 2015, to either commit an act of terror or to wait for further instructions, the prosecutor's office said. They were charged with being members of a foreign terrorist organization.
One of the accused, a 17-year-old that CNN is not naming, joined ISIS at the end of September 2015 in Raqqa, the terror organization's de facto capital in Syria. He is suspected of receiving training there, including the handling of weapons and explosives. In October 2015, the three accused pledged to go to Europe, the prosecutor's office alleges.
They were given passports provided by ISIS and received thousands of US dollars as well as mobile phones.
The suspect who investigators believe had links to the network behind the August 2015 train attack also had contacts in the Netherlands, including some who offered help with logistics, the source said without elaborating.
Details about that suspect's alleged links to people behind the train attack weren't immediately available.
In that incident, a Moroccan national, Ayoub el-Khazzani, attempted to launch a gun attack on a high-speed Thalys passenger train on the Belgian-French border, but the attack was thwarted by three Americans. Khazzani was subdued and arrested
Khazzani was connected through associates to Paris attack coordinator Abdelhamid Abaaoud, according to French investigative documents obtained by CNN. Abaaoud was killed in a police raid on a Saint-Denis apartment
days after the Paris attacks.
According to a senior European counter-terrorism official, investigations into the Paris attack network have produced indications that ISIS had a broader set of potential targets
-- including the Netherlands -- when it dispatched operatives to Europe last autumn.
ISIS at large
European officials told CNN they believe ISIS is ratcheting up its planning for international attacks to retaliate for losses in Syria, Iraq and Libya.
In 2015 and 2016, several European countries have experienced atrocities, including the Paris attacks
, that were linked to ISIS-affiliated terrorists and "lone wolf" actors who have pledged loyalty to the jihadist group.
French Prime Minister Manuel Valls warned Sunday
that France remains a target for terror and the country will suffer new attacks.
"The threat is maximal," Valls said in an interview with the Europe 1 radio station. "We have seen it again in the past few days, the past few hours, and even as we speak. Every day intelligence services, police and gendarmerie thwart attacks and dismantle Iraqi-Syrian networks."