The brothers, aged 15, 16 and 20, were attacked as they were walking home in Victoria Street, Dundee, close to the Skyaxe kickboxing club between 12.30am and 2am last Wednesday.
The 16-year-old was taken to Ninewells Hospital in the city for treatment to a broken wrist and injuries to his face.
The 20-year-old brother sustained a broken finger and cuts to his face and the 15-year-old suffered a number of cuts and bruises, police said.
The attackers are believed to have been in a group of about 12 people in the area, all aged in their late 20s or early 30s.
Two of the attackers were men wearing kilts, white shirts and black waistcoats, one had blonde hair and the other had black hair.
Police are appealing for any witnesses to the attack to contact them and they are particularly keen to hear from a woman who was in the area at the time.
She is in her late teens with dark hair tied back in a ponytail and was wearing a light-coloured jacket.