The 21-year-old victim was taken to hospital for treatment for injuries to his arm, believed to have been caused by a knife in the early hours of Sunday.
Police were called to a disturbance in Newport's High Street at about 12.45am.
A 33-year-old man and a 48-year-old man, both from Newport, were arrested on suspicion of causing grievous bodily harm, alongside a 17-year-old boy.
All three are on bail while the investigation continues. The victim's injuries are not thought to be life-threatening.
Detective Sergeant Andrew Dawson said: “We understand this will be concerning for local residents and we would like to reassure the community that we are taking this incident extremely seriously.”