The victims were on their way from Haldwani in Nainital to Berinag in Pithoragarh district when the mishap occurred near Gainar-Chunar bend, about 35 km from the district headquarters, killing three persons on the spot and leaving eight others injured, police said.
The injured have been rushed to a nearby hospital.
The cause of the mishap is not yet known, they said.
Governor K K Paul expressed grief at the loss of lives in the incident. He also expressed concern over the frequent accidents on the state's roads.