Two boys have been arrested on suspicion of murdering an 18-year-old man, police said.
Aqeel Khan was found with suspected stab wounds in Watford yesterday afternoon.
Two boys, aged 15 and 16, were arrested overnight on suspicion of murder.
A 21-year-old man was arrested in connection with the incident yesterday afternoon.
All three men, from Watford, remain in police custody.
Mr Khan was found in Croxley View at around 4.20pm and was taken to Watford General Hospital where he died a short time later.
Detective Inspector Jon McAdam from the Bedfordshire, Cambridgeshire and Hertfordshire Major Crime Unit said: "Our investigations into this incident are continuing and further arrests have now been made.
"Aqeel's family are being supported by specialist family liaison officers at this difficult time and I extend my condolences to them.
"I would like to renew my appeal for information. If you believe you witnessed the events yesterday afternoon, or have information which could assist the investigation, please contact police."
Nick Lillitou, Safer Neighbourhood Inspector for Watford, said: "This is an isolated incident for Watford. It is not a common occurrence in the town and I'm sure the community of Watford will have concerns about what happened.
"We will have additional officers carrying out extra patrols in the area to provide reassurance to local people. If you do have any concerns or information you would like to share please do speak to these officers."
Anyone with information is asked to contact police via the non-emergency number, 101 or Crimestoppers, anonymously, on 0800 555 111.