The incident took place yesterday when Ankit Srivastava (45), his son Ani (3) and his brother-in-law Rajkumar (35) and three others were on their way to Nainital, they said.
Ankit, Ani and Rajkumar were killed while Ankit's wife Anamika, Rajkumar's wife Nirmala and son Ayush were injured. They have been admitted to a nearby hospital.
A probe has been launched into the matter but no arrests have been made so far, the police said.