The case of the 13-year-old boy, identified only by his surname Wang, has sparked anger and furious debate on juvenile crime.
Authorities found his remains buried in an abandoned vegetable garden.
The boy was bullied in school, according to his father.
Police are investigating the case as an intentional homicide, state-run Global Times said.
All three detained teens are under 14. Under Chinese law, those above 12 years of age but under 14 can face criminal prosecution only when allowed by the Supreme People's Procuratorate, the country's highest public prosecutor.
The victim and his classmates are "left-behind children", a term used to describe kids in China who live with their grandparents in rural areas while their parents work in the cities.
The case has triggered an outpouring of grief on social media, with tens of thousands of posts on Weibo and Douyin over the weekend.
"The whole country is watching this case, hope the police can be fair and give the victim's family a satisfying answer," one user said in a comment on a post by the victim's father, which got 50,000 likes.
Many in the comments said that the suspects should be punished harshly despite their age.
The victim went missing in the afternoon of 10 March, his father said in a video on Douyin, China's version of TikTok. The local government said the boy likely died on the same day.
"My child was still alive and kicking around 15:00 on 10 March... All his money was transferred from his phone at 16:10 and his phone was turned off," the father said.
Before his death, the boy transferred 191 yuan ($17; £13) to one of his three classmates, the father told The Beijing News.
He had been "bullied by (his) three classmates for a long time", said the family's lawyer, Zang Fanqing, in a Weibo post on Monday.
Under questioning by authorities, the three led police to the boy's body, after initially denying that they knew where it was buried.
Police are still investigating the motive for the killing, The Beijing News reported.
-- Courtesy of BBC News