The girl was from a south-eastern province bordering Vietnam, where the disease has killed three dozen people.
An epidemiologist for the World Health Organisation confirmed the girl had died from the H5N1 strain, as already announced by local health officials.
The previous Cambodian victims were a 28-year-old man and a woman aged 25.
The WHO's Megge Miller said the girl's death was still being investigated by officials sent to her village in order to establish how she contracted the virus.
"We're still looking into it and looking at the exact timing to see how she got sick," Ms Miller said.
The H5N1 virus has killed 36 people in Vietnam and 12 in Thailand since December 2003.
The three Cambodian victims were all from Kampot province.
A local health official who asked to remain anonymous told AFP news agency that although chickens had fallen ill in a village there, villagers were reluctant to let the authorities cull their birds.
World health experts have repeatedly warned that the H5N1 virus could easily mutate, leading to a deadly pandemic.
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