The oldest of the accused, identified as 27-year-old Mohamed A., was given the longest sentence of six-and-a-half years by Hamburg's higher regional court, German news agency DPA and public broadcaster NDR reported.
His 19- and 20-year-old co-accused, minors at the time of the acts, received prison terms of three and a half years each.
Judges at the Hamburg court said the IS jihadist group had arranged for the trio to travel from Syria to Germany in 2015, where they were told to await orders for an attack.
No evidence was found however of concrete plans for a terror plot.
To reach Germany, the men travelled via Turkey and Greece — using the same route as hundreds of thousands of refugees fleeing war-torn Syria at the time.
The accused were given fake passports, cash and mobile phones from IS, according to the court, and used the same smuggling network as the assailants who had carried out the IS-claimed attacks in Paris in 2015 that left 130 people dead.
There were “many parallels” between the German “sleeper cell” and the Paris cell, the presiding judge said, according to NDR.
The three Syrians were arrested at different refugee shelters in the northern state of Schleswig-Holstein in 2016.
The 27-year-old had admitted during the trial last month that he was a member of IS. His co-accused made no confessions.