Fahad Mohamed Nur died after being attacked near Cathays Railway Station.
A 21-year-old from Cardiff arrested in London on suspicion of murder has been brought back to South Wales for questioning.
Two other men, aged 24 and 22, from Cardiff, previously arrested on suspicion of murder, and a 34-year-old woman from Birmingham arrested on suspicion of assisting an offender, remain in custody.
Fahad’s family have been updated and continue to be supported by specially-trained family liaison officers.
It is believe Fahad was known to his attackers.
Senior Investigating Officer, Detective Chief Inspector Mark O’Shea, from South Wales Police, said: “Three men are now in custody on suspicion of murder. We would very much like to thank the community for the support and information that has already been provided and would urge anyone else with information to please contact us.”
Officers were called to the lane which links Park Place to Corbett Road at approximately 00:24hrs on Sunday, June 2 following reports that a man had been stabbed.
Fahad, 18, was taken to the University Hospital of Wales, Cardiff, where he died from his injuries.
An incident room has been set up in Cardiff Central Police Station and a team of detectives are working on the investigation.
“Fahad comes from a loving family who are devastated by what has happened, and their grief is shared by the wider Somali community who are proving to be an invaluable support,” added DCI O’Shea.
Information and digital footage can be provided online via a public portal https://mipp.police.uk/operation/62SWP19A85-PO1
Alternatively, please contact South Wales Police via 101 or Crimestoppers anonymously on 0800 555 111 quoting reference number *198071.