The video filmed on Sunday in the southeastern town of Fontaine shows two of the boys pushing the 18-year-old in a park, making him fall and shouting at him before shoving him into a stream, from which the bewildered-looking victim emerges and walks away.
According to the police source, a 14-year-old and 15-year-old were detained yesterday over the assault, and two other boys were held today.
One of the boys, a 12-year-old, was released and sent home after being questioned.
French prosecutors are due to question the three teenagers later today.
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The outrage over the filmed incident follows a similar outcry over a video posted on the Internet showing a 24-year-old Frenchman hurling a kitten through the air.
Farid Ghilas was arrested in the southern city of Marseille last week and sentenced to a year in prison yesterday.
The five-month-old kitten, shown mewling and traumatised in the video, survived but suffered a broken leg and is facing surgery.