At least two people were seriously ill with the disease, which can kill within 24 hours, and one of them was "close to death", Chinese health officials said.
Seven others were described as "stable" and authorities were tracking down people who had been in contact with those infected.
Police were put on guard outside the hospital treating the infected.
Residents are reported to have fled the area, despite the quarantine.
Xiumaocuo, a Tibetan construction worker from another village in Qinghai, said many migrant workers had already gone back to their hometowns.
"My boss told me that more than 50 of the 100 construction workers on our project left homes already," she said.
Some people also tried to leave the quarantined area on Monday evening, mostly on foot, after the third death was reported, two residents told The Associated Press news agency.
Chinese media reported the latest victim of the plague ws a 64-year-old herdsman who became ill after burying his dog which had died suddenly.
The pneumonic plague outbreak in Ziketan, was first detected on July 30.
China has had previous cases of plague, and experts say most cases in the northwestern region occur when hunters are contaminated while skinning infected animals.
Liang Wannian, a rodent plague expert, said the move to prevent the spread of the disease had been successful.
"The plague is under control, and our work can be seen as effective," he said.
Pneumonic plague is the least common and most deadly form of the disease, and the airborne bacteria can be directly transmitted between humans through inhalation.
The highly-infectious disease circulates mainly among small animals like rats and mice, but can also infect humans directly through close contact.
It is curable if treated early with antibiotics.
The World Health Organisation said in a 2006 report that most cases in China's northwest occur when hunters are contaminated while skinning infected animals.
Worldwide, thousands of plague cases are reported each year, mostly in Africa.
Between 1998 and 2008, nearly 24,000 cases were reported, including about 2,000 deaths, in Africa, Asia, the Americas and eastern Europe.