The decomposing bodies of 71 people, including a baby girl, were found in an abandoned truck in Austria close to the border with Hungary, Austrian officials said on Friday.
Hungarian police said the truck driver had been arrested.
Austrian police said of the 71 dead, 59 were men, eight women and four children including a baby girl, The Guardian reported.
"We currently have three people under arrest in Hungary... and expect that this is the trace that will lead us to the perpetrators," a police officer said, making clear the people being held were not ring leaders.
The death toll was raised on Friday from initial estimates of 20 to 50.
The truck, abandoned on a road near Parndorf, had apparently been there since Wednesday.
Austrian police said all those on board appeared to have suffocated and died before they entered the country.
The truck had the logo of a Slovakian poultry company, Hyza, which said it no longer owned the vehicle but the buyers had not removed the branding.
