The incident occurred late on Tuesday night when a verbal spat broke out between Anand (40), his wife Priti (38) and Anand's cousin brother Rupesh (29) following which all of them consumed poison. Rupesh, however, survived, SP Rajesh Singh said.
The matter came to light yesterday when Rupesh's father found all of them in an unconscious state and rushed them to a nearby hospital in Agra.
"Primary investigation reveals that Priti and Rupesh had illicit relations for almost five years. Rupesh was married earlier this month and his wife Pooja also came to know about this and returned to her parents' house," he said.
The bodies were handed over to family members after postmortem yesterday.