The team of firefighters had responded after midnight to an alert about a gas leak in the small town of Quargnento, in Piedmont.
As the firefighters attempted to put out a fire in an abandoned farm building, a second explosion occurred, bringing down the entire structure and killing the three men.
In a nearby building, authorities found an unexploded gas cylinder, a timer, and electrical wire, the local prosecutor said.
“All this leads us to believe that the explosion was intended and deliberately set,” local prosecutor Enrico Cieri said.
Sky TG24 reported that the timer was remote-controlled, and that the owner of the farm building, which had been renovated and has been up for sale for two years, had been questioned.
Two other firefighters and a member of Italy’s Carabinieri law enforcement corps were injured in the incident.
Those killed were aged 32, 38 and 46, according to local media.
Local press reports said investigators were looking into possible insurance fraud or a family dispute behind the inferno.
Italy’s interior minister, Luciana Lamorgese, visited the site, amid an outpouring of sympathy for the families of the dead and injured from Italian officials and authorities.
Lamorgese declined to comment on the investigation.