Another person in ethnic Tibetan parts of China has burned himself to death in protest against Beijing's rule, activists say, a day after five similar self-immolations were reported.
They say large crowds of Tibetans later gathered at the scene in Qinghai province, amid heavy police presence.
On Wednesday, five people reportedly set themselves on fire, and at least two of them later died.
China accuses exiled Tibetan leaders of stirring up unrest.
The government in Beijing says Tibetans have religious freedom, blaming Tibet's exiled spiritual leader, the Dalai Lama, for the unrest - a claim he rejects.
More than 60 Tibetans have set themselves on fire since early 2011.
The latest incidents took place as China's Communist Party opened a congress in Beijing that will begin the process of transferring power to a new generation of leaders.
On Thursday, Tibetan nomad Jinpa Gyatso, 18, set himself on fire in the square outside a monastery in the western town of Rebkong, Tibetan activists in exile say.
He was reportedly heard shouting for the return of the Dalai Lama to Tibet before he died.
On Wednesday, Dicki Chhoyang, a spokesperson for the Tibetan government in exile in India, told Agence-France Presse news agency that their sources were able to confirm the four cases, calling them a "cry for help".
The London-based Free Tibet group said that the three boys set themselves on fire outside their local police station.
It said it was the first documented case of a triple self-immolation.
The youngest, aged 15, died at the scene. The whereabouts of his two 16-year-old companions are unknown, a group spokesman said.
The woman was identified by Voice of America as 23-year-old Tamding Tso. She had a five-year-old son, reports said.
Radio Free Asia also reported a fifth self-immolation inside Tibet, citing an exiled monk who had spoken to people in the region, however this could not be confirmed.
Foreign media are banned from the region, making verifying the self-immolation cases difficult. Chinese state media have confirmed some but not all.
More than half of those who set themselves on fire are believed to have died.
Last week UN rights chief Navi Pillay called on China to address issues behind the series of self-immolations and to allow monitors and the media access to the region.