The accident occurred when the victims were returning to their house in Narwal village in the district, a police officer said.
The deceased were identified as Mishri Lal (40), his wife Neelam (34), and his mother-in-law Savitri Devi (57), Ajeet Singh Chauhan, police circle officer (Cantonment) said.
The driver of the truck was arrested and a case under relevant sections of the Indian Penal Code has been registered against him, he said.
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