The figure includes an estimated 143 million mammals, 2.46 billion reptiles, 180 million birds, and 51 million frogs, a report commissioned by the World Wide Fund for Nature (WWF) found.
The number of reptiles is significantly higher than the others because there are generally more of them per hectare (10,000 square meters) than mammals or birds.
"The interim findings are shocking. It's hard to think of another event anywhere in the world in living memory that has killed or displaced that many animals," said WWF-Australia CEO Dermot O'Gorman. "This ranks as one of the worst wildlife disasters in modern history."
With more than 15,000 fires across every Australian state, this was the worst fire season on record, according to the report.
Researchers are still working to finalize the report, titled "Australia's 2019-2020 Bushfires: The Wildlife Toll," but the three billion figure is not likely to change, according to a WWF statement.
The research was carried out by scientists from the University of Sydney, University of New South Wales, University of Newcastle, Charles Sturt University, and BirdLife Australia.