Ram Sagar, 27, was set ablaze in Saraibibi village of Sitapur district, about 80 km from here on Wednesday.
"Raj Kishore, Sushil and Rajesh of the same village took Sagar along with them and set him on fire after pouring kerosene on him," police inspector R.P. Shahi said.
He said Sagar threatened to expose the three men after they stole a hand pump from a brick kiln. All the four men were employed in the brick kiln in Ludahaura village of Sitapur.
Sagar, who was undergoing treatment in a government-run hospital, succumbed to his injuries on Thursday.
The three suspects are absconding.