Police said the incident happened on the highway from Udon Thani to Phitsanulok in Loei province near Kilometre 22 Friday afternoon.
The bus with about 50 passengers was running downhill when a pickup truck coming on opposition direction into the lane of the bus.
The truck hit the side of the bus while it swerved to avoid collision causing the bus driver to lose control.
The uncontrolled bus ran down 500 metres and rammed at power poles before overturning and falling into the roadside ravine.
The three deaths included a child, and 43 were injured, 23 seriously.