They then bundled Singh into a car and sped away.
Later, Singh's body was found near Sheikhupura district, 40 km from Lahore.
The body bore marks of torture, police said.
Singh was a 'hakeem' and dealt in natural medicines.
When police contacted Singh's family in the northwestern city of Peshawar, they learnt that he had recently decided to marry for a second time despite opposition from the woman's fiance.
"The deceased's family told us that Amerjeet Singh had contracted a second marriage with the fiance of Arjun Singh. It suspected that Arjun and the brothers of the second wife of Amerjeet might be involved in this murder," an official of Shalimar police station told PTI.
A murder case had been registered against Arjun Singh, Saroop Singh and Mastan Singh, the officials said. Singh's body was sent to the city mortuary for an autopsy.