The accident took place this morning when the tractor driver Sunil (30) lost control of the vehicle attempting to save a stray dog on the road and it overturned near Nibahra village, they said.
The other two deceased were identified as Teerath (35) and Neeraj (18).
The tractor which was carrying wheat husk in the trolley to Sisaiya crossing for sale, has been seized, police said.
The injured has been admitted to the hospital where his condition was stated to be stable, they added.