Detectives are seeking a 'mystery victim' of an assault in Linden Road, between 4.30pm and 5.30pm last Monday (May 17), Sussex Police said.
Police said the 'as yet unidentified victim' — a boy described as white, aged 15 to 16, with light brown hair, wearing a red sports style jacket and jeans — was 'pushed and spat at'.
The suspects are a group of boys all aged between 13 to 16, wearing tracksuits and medical face masks, police said.
A spokesperson added: "A second victim, a 29-year-old man, became involved solely as a result of asking if this unidentified boy was okay, and was then attacked by the youths as a result, as the boy disappeared. Police are not aware at this stage of any injuries either of them sustained.
"Police are asking the boy who was first assaulted to get in touch so that officers can establish a complete picture of what happened.
"If you are that boy or if you know who he is, please contact Sussex Police online or by calling 101, quoting serial 1141 of 17/05."
Police confirmed that three boys, two aged 15 and one aged 17, were arrested on suspicion of affray and assault involving the two victims, assault on police and resisting arrest.
"After being interviewed they were all charged with resisting arrest, assault on police, and breach of Community Protection Notices," the spokesperson said.
"The investigation into the attacks on the two victims continues."