Two other workers were left clinging to the structure suspended high above the Bangkok streets in the accident at the Baiyoke tower on Monday afternoon.
"Three men fell to their deaths... while another two were rescued from the 68th storey -- probably one floor lower than where they had been working," police lieutenant colonel Thamarak Reungdit told AFP.
He said the two survivors suffered minor injuries and had been treated in hospital. The bodies of the victims were recovered from terraces on different storeys of the building.
"A forensic team will investigate how it happened and whether or not the platform was overloaded," he said.
The 304 metre (997 feet) Baiyoke tower is a distinctive feature of the Thai capital's skyline and houses a hotel on its uppermost floors.