A police spokesman said three teenagers were arrested after a boy was threatened at knifepoint at the Manor Sports Ground in Broadwater Road, Worthing, between 5.30pm and 6pm on Sunday (March 10).
The spokesman said two suspects approached the 13-year-old and, after engaging him in conversation, drew him away from the car park before threatening him with a knife and trying to steal his backpack.
The boy managed to escape with minor injuries, police said, but had to leave his black Scott bicycle behind, which has not yet been found.
A 14-year-old boy from Sompting and two 15-year-old boys from Lancing and Worthing were arrested on suspicion of robbery, police added. They have been released under investigation.
The spokesman said: "Anyone who witnessed the incident, who may have seen three boys acting suspiciously in the area in the time leading up to it or know the whereabouts of the bicycle is asked to report online or call 101 quoting serial 1259 of 10/03."