Sussex Police said the boy, 14, had been at the Lyons Farm recreation ground when he was attacked between 11pm and midnight last night (July 16).
According to police, he suffered head injuries and concussion and was taken to hospital for treatment, but has since been released.
Police said a 16-year-old boy was arrested on suspicion of wounding with intent; a 16-year-old girl was arrested on suspicion of assault by beating, attempt to cause grievous bodily harm with intent and handling stolen goods; and a 14-year-old boy was arrested on suspicion of grievous bodily harm with intent and affray.
All three, from Worthing, remained in custody at 4pm today (July 17).
A spokesman said: "Police are appealing for any witnesses to the incident or who may have seen anyone in the park in the time leading up to it to report online or call 101 quoting serial 83 of 17/07."