The incident took place when Ghatampur resident Saurabh was going to Duraili with his sister-in-law Meenakshi (28) and nephew Astitva (4)on bike yesterday evening, police said.
Saurabh, who was riding the bike, is undergoing treatment after sustaining serious injuries, they said.
The bodies have been sent for post-mortem and a manhunt has been launched to nab the absconding driver of the tanker, they said.