The bodies of Mansij (7) and his sister Yashbi (5) were found near Pragati Maidan yesterday morning, four days after they were kidnapped from their school in east Delhi's Mandawli area.
The gang had tried to kidnap the children on a previous occasion on Feb 23 but could not do so as they did not go to school that day.
They even allegedly planned it to be that perfect that they locked the door of the victims' house so that their mother could not go out to pick them up from school.
They allegedly strangled the children and smashed their heads with rocks on the same day they kidnapped them after they found that it was difficult to keep the siblings with them.
"We have arrested Amit Singh (22), the uncle, Shivam Gupta (19) and Pankaj Kashyap (19). The trio wanted to make fast money and knew that the family had got around Rs 30 lakh in a property deal," Prabhakar, Deputy Commissioner of Police (East), said.
Another accomplice Abhilash is on the run.
"Singh had scouted a deal for the sale of a flat in Mandawali and needed Rs six lakh immediately for clinching it. Kashyap on the other hand had lost heavily in gambling and had run debts amounting to Rs 1.50 lakh and needed money to repay the debts. Gupta and Abhishek needed money to satisfy their consumerists instincts," Prabhakar said.
Kashyap, Gupta and Abhishek were to get a share of Rs two lakh each and the rest was to be kept by Singh, he claimed.
Ransom calls made from telephone booths near the victims' family gave police the first headway and suspicion fell on Singh, who was trying to raise money for a property deal.
After Singh "confessed" to the crime, he and his two accomplices were arrested, the DCP said.