The court ruled that the three were all high-ranking members in the outlawed Revolutionary People's Liberation Party-Front (DHKP-C), a group which investigators say seeks to overthrow the present Turkish government and replace it with a Marxist one.
Membership in a foreign terror organization carries a maximum sentence of 10 years in prison.
The group features on the terror lists of Turkey, the European Union and the United States.
As part of a plea agreement in which they confessed to involvement with the group, Hasan S. (46) was sentenced to two years and 11 months in prison. Mustafa A. (52) and Ilhan D. (40) who were also convicted of forging documents, were given five years and three-and-a-half years respectively. Hasan S. and Mustafa A. are Turkish nationals. Ilhan D. is a German national of Turkish origin.
After the sentencing, Mustafa A. and Ilhan D. walked free from the courtroom as they had already served their time in detention before and during the trial which lasted 15 months.
The DHKP-C is believed to have orchestrated attacks on Turkish institutions in Germany as well as on purported "enemies" of the group until it was banned in August 1998.
The following year, the group's general secretary, Dursan Karatas, said that it would no longer use violence to achieve its political aims.
The DHKP-C is also accused of masterminding a wave of hunger strikes among left-wing prison inmates and their friends and families in Turkey that has resulted in nearly 70 deaths in four years.
Convictions follow extradition of regional leader
The convictions in Stuttgart come after Hasan Huseyin K., the suspected leader of a regional chapter of the DHKP-C in Germany, was extradited to Germany from the Netherlands in June.
He was accused of belonging to a terrorist organization and also faced charges of extortion and of beating and shooting two political dissidents in a Hamburg restaurant.
The DHKP-C is believed to have some 650 members and supporters in Germany.
Editor: Susan Houlton