The truck, believed to be carrying stolen logs, was moving on Umrangso-Meghalaya Road when it overturned as the driver lost control, police said.
The three were identified as Itom Sinha (32), Abdul Rauf (33) and Abdul Karim (35), while the injured are Babu Barman (43), Lokendra Kemprai (42) and Said Ali (29).
The injured have been admitted at Umrangso Hospital and the bodies sent to Haflong Civil Hospital for post-mortem.