Police are now appealing for witnesses to the serious assault, which took place on Saturday (July 24) at around 12.10am
Officers were on patrol when they came across an altercation at the junction of Harold Place and Robertson Street.
Police said the majority of the group dispersed upon police arrival.
Officers found one man unconscious on the ground with a head injury. He was taken to the Conquest Hospital for treatment.
Police said CCTV enquiries found the victim had been talking to a group of men outside Yates minutes before the assault, and it is believed these men were involved in the altercation.
One suspect is described as being around 6ft 2ins tall and wearing a light blue hooded jumper with HOODRICH written across the front.
Police said a second suspect is described as being shorter and of stocky build, with his hair styled in a black quiff and wearing jeans and a blue sweatshirt.
The third suspect is described as being of slim build.
Anyone who saw what happened, or who may have any other information, is asked to contact police online or by calling 101 quoting serial 19 of 24/07. Alternatively, you can contact the independent charity Crimestoppers anonymously on 0800 555 111.