Colonel Hoang Van Man, head of the Quang Nam Border Guard, said the boat owned by Tran Van Cong, 47, of neighboring Quang Ngai Province had gone out to sea Saturday afternoon with eight men.
At around 3:30 a.m. on Monday, when it was in waters off Thang Binh District, it was hit by a boat whose identity is not known yet.
It caused all eight to fall overboard.
The Border Guard dispatched 10 men to the scene and asked for help from fishermen nearby to rescue them.
Man said: "All eight have been found but three died. The other five have been brought back to shore."
The Border Guard is working with relevant authorities to investigate.