The three extremists, all 16 at the time, allegedly set off a home-made bomb in April outside a prayer hall where a Sikh wedding was taking place, also leaving two others with lesser injuries.
The youths, who were born in Germany, had met on social media after being radicalised by ultra-extreme Salafist Muslims, and got together to commit the attack in the western city of Essen.
Prosecutors charge they had sought to kill "non-believers" with a device they had built from a fire extinguisher shell and chemicals ordered from online shopping site Amazon.
Neriman Yaman, mother of one of the accused, the now 17-year-old Yusuf, has described her torment as she watched her young son become radicalised in the book "My Son, the Salafist".
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The Turkish-born author describes how she sought help from mosques and state authorities as her son from age 14 turned to a fundamentalist version of Islam.
She says Yusuf started speaking in verses, watched Arabic-language preachers online, joined a group that handed out Korans on the street, and married a teenage girl wearing a burqa.
"As parents we were powerless," she later told journalists.
"The other side was stronger than us ... The only comfort in this huge misfortune is that no-one has died.