The accident took place in the Thung Khe Pass in Mai Chau District's Tong Dau Commune.
At around 2 a.m. Nguyen Dinh Luc, 42, was driving the bus from Hanoi to Son La Province when he plowed into the motorbikes from behind.
The bus then hit the side of the mountain and flipped over.
Three people on the motorbikes were killed instantly and around 20 passengers in the bus were injured, some critically.
Ha Van Tich, chairman of Tong Dau Commune, said the deceased people were migrant workers on their way home for the Lunar New Year.