The incident took place in Mohammadpur locality under town police station of Begusarai district late last night, Deputy Superintendent of Police Rajesh Kumar said.
36-year old Amit Kumar, his 30 year old wife Priyanka Kumari and three-year old son Aryan were sleeping on the roof of the house when miscreants attacked them and slit their throats with sharp weapons, the DSP said.
The victim was engaged in land business and also ran a coaching centre, he added.