All three victims tested positive for the H5N1 strain of the virus and died between July 30 and Aug. 3, said Trinh Quan Huan, head of the Ministry of Health's Department for Preventative Medicine and HIV/AIDS Control.
Two children, ages 4 and 1, died in Ha Tay province about 30 miles west of Hanoi, while the other victim died in southern Hau Giang province in the Mekong Delta, about 110 miles south of Ho Chi Minh City, he said.
No other details were available about the third victim or how the three were infected.
The deaths follow an outbreak earlier this year that swept through Asia, killing 16 people in Vietnam and eight in Thailand and prompting the slaughter of about 100 million poultry across the region.
Deputy Prime Minister Nguyen Tan Dung sent a telegram to all relevant agencies and provinces in Vietnam on Thursday urging people to be vigilant against the disease, Huan said.
In recent months, bird flu has reappeared in China, Thailand, Indonesia and Vietnam, which had declared itself free of the disease at the end of March. The last previously confirmed bird flu death in Vietnam was a 12-year-old boy who died March 15.
Vietnamese officials have been struggling to eradicate small outbreaks of the disease, which has resulted in the deaths or slaughter of more than 50,000 birds.
Health authorities had warned after the earlier outbreak that the disease could return with warmer weather.
"It's not surprising that it has come back," Roy Wadia of the World Health Organization said last month in Beijing. "It stays in the environment a long time."
By Margie Mason