A spokesman for Sussex Police said the 13-year-old victim was kicked off his bike and threatened at knifepoint in Broadwater Road just before 6pm on Sunday evening (March 10).
Three boys were stopped nearby and detained by police officers at about 6.45pm, said police.
A 14-year-old boy from Sompting and two 15-year-old boys from Lancing and Worthing were arrested on suspicion of robbery, said police, and have been released under investigation.
The spokesman said: "Anyone who witnessed the incident or who may have seen three boys acting suspiciously in the area in the time leading up to it is asked to report online or call 101 quoting serial 1259 of 10/03.
"If you see anyone with a knife in the street, please dial 999 immediately."