The Anatolian state news agency named the dead child as 11-year-old Hulya Kocyigit. Hulya was the sister of Mehmet Ali, 14, who died last weekend, and of Fatma, 15, who died on Thursday.
Turkey's health ministry said on Friday the World Health Organisation (WHO) had confirmed the children had contracted the bird flu virus.
Necdet Unuvar, a senior health ministry official, told a televised news conference: "The test results for the three came back positive... They have been confirmed by the WHO laboratory."
The children lived in Dogubayazit, a town in the remote rural district of eastern Turkey near the Armenian border. A six-year-old brother is also being treated for the same disease in the hospital.
A hospital official said another 25 people or so from the Dogubayazit area were being treated for possible bird flu symptoms on Friday.
According to reports from the Associated Press, health and agriculture authorities were attempting to manage more outbreaks across the east of Turkey.
The hospital official, speaking on condition of anonymity, said all the patients were being treated at the 100th Yil Hospital in Van, about 200km from Dogubayazit. One was on life support. Another 25 to 30 people had come in for blood tests, received medical care and left.
The authorities are closely monitoring the H5N1 strain of the virus, fearing that it could mutate into a form easily passed between humans and spark a pandemic killing tens of millions.
On Thursday, Mehdi Eker, the agriculture minister, said new bird flu cases were detected in five areas in eastern and southeastern Turkey and authorities have culled 7000 fowl in those areas.
In Istanbul, authorities took four pigeons, found dead near a mosque, to a laboratory for bird flu tests and sprayed the area with disinfectants, according to an official statement on Thursday.
In October, Turkey successfully contained an outbreak of the deadly H5N1 strain of the virus in the western town of Kiziksa, after culling more than 10,000 fowl.
According to Eker the task is harder in eastern Turkey where almost every household has some fowl and they allow them to go inside their houses at night.
Children are among those most
at risk from a possible pandemic
According to reports, the Kocyigit family took their fowl inside the house when temperatures fell at night and killed and ate the chickens when they got sick.
The siblings from the eastern town of Dogubayazit were hospitalised last week after developing high fevers, coughing and bleeding in their throats. It was also reported that they had played with the heads of the dead chickens.
Medicines were flown into Van on Thursday as the authorities mobilised to deal with the outbreak. A team of officials from the WHO and the Food and Agriculture Organisation was also sent to Turkey.