The mob attacked the trio at their residence at Smit village, about 8 km from here, last night after a person from the same locality alleged that the trio indulged in witchcraft rituals last week, they said.
"Three persons were killed and one of our constables was injured in the incident last night," East Khasi Hills Superintendent of Police M Kharkrang said.
The victims were identified as Yesly Mawthoh and his son D Nongdhar and Besly Mawthoh, all from the same family, the SP said.
The unidentified person was rushed in an unconscious state to a private hospital with injuries on his nose and nails.
He had named the trio shortly after he gained consciousness yesterday after which an angry mob rushed to the residence of the trio, they said
On being informed of the attack, police teams rushed to the spot, baton-charged the crowd who were witness to the incident in which one of the police constables was also injured in the melee.