A man believed to be in his 50s, died in the blaze at 0229 GMT on Saturday in Loudoun Square, Butetown, Cardiff.
The fire is not being treated as suspicious and police applauded the three men who became involved.
They want the trio to urgently contact officers at Cardiff Bay who are investigating the incident.
The men are believed to be local.
Det Sgt Ceri Hughes, who is leading the investigation, said: "We want to praise these men for the way they helped and are asking them to get in touch.
"Although the fire is not being treated as suspicious we are still investigating and these men may have crucial information to help us establish the circumstances of what's happened."
An investigation is being carried out to find out the cause of the blaze.
Anyone with information is asked to contact Cardiff Bay CID on 02920 222111 on extension 34420 or Crimestoppers on 0800 555111.
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