The two men and a woman are being sought in connection with the attack on the 17-year-old Kurdish Iranian who waiting at a bus stop with two friends outside The Goat pub in Croydon, south-east London.
The local MP, Gavin Barwell, has described the attackers as "scum".
The Metropolitan Police said they know "approximately 20" were involved the incident that left the boy with a fractured skull and a blood clot on his brain.
On Monday police renewed their appeal for witnesses and information as the victim remained in hospital in a serious but stable condition.