The disease sweeping West Africa has also touched Liberia, Guinea, Nigeria and Senegal and is believed to have sickened more than 5,300 people, the World Health Organization reported. In a sign the crisis is picking up steam, more than 700 of those cases were recorded in the last week for which data is available.
Ebola is estimated to have killed more than 2,600 people, with most of the deaths in Liberia. But WHO has said that the official toll is probably a gross underestimate and that most patients are at home, infecting others, when they should be in treatment centres.
The U.N. Security Council was scheduled to discuss the crisis later Thursday.
During the lockdown, which was set to begin at midnight Thursday and run through Sunday, volunteers will try to identify sick people reluctant or unable to seek treatment. They will also hand out 1.5 million bars of soap and deliver information on how to prevent Ebola.
More than six months into the world's largest Ebola outbreak, there are still affected areas without access to water or soap, WHO said.
Authorities have said they expect to discover hundreds of new cases during the shutdown. Many of those infected have not sought treatment out of fear that hospitals are merely places people go to die. Others have been turned away by centres overwhelmed with patients.
Sierra Leone's government said it has prepared screening and treatment centres to accept the expected influx of patients after the shutdown.
As shoppers rushed to buy last-minute items, some merchants worried about how they would feed their own families after losing three days' income. Much of Sierra Leone's population lives on $2 a day or less, and making ends meet is a day-to-day struggle.
"If we do not sell here we cannot eat," said Isatu Sesay, a vegetable seller in the capital. "We do not know how we will survive during the three-day shutdown."
Several countries have promised aid. France announced Thursday it will set up a military hospital in Guinea in the coming days, while Britain said it will provide 500 more badly needed beds in Sierra Leone. The U.S. plans to send 3,000 military personnel to the region and build more than a dozen treatment centres in Liberia. An American general has arrived in the Liberian capital of Monrovia to set up a command centre.
Ebola, which is spread through bodily fluids, puts health workers at a particularly high risk. Nearly 320 have become infected, and about half have died. A French nurse for Doctors Without Borders who became infected in Liberia was being flown to Paris on Thursday.
With no proven treatment for Ebola, public health experts have kept the focus on isolating the sick, tracking down those they have come into contact with, and stopping the chain of transmission through travel restrictions, the cordoning off of entire communities and now Sierra Leone's lockdown.
Confusion and fear about the disease and anger over some of these measures have occasionally sparked unrest. In Guinea this week, a team that was doing disinfection and education on prevention methods was attacked by a group of young people and has been missing ever since.
Some patients have been given the blood of Ebola survivors in an experimental approach that some scientists think can help people fight off the virus.
British nurse William Pooley, who was infected while working in Sierra Leone and has since recovered, has flown to the U.S. to donate blood to an American patient, according to the Foreign Office. The American was not identified.
Associated Press writers Maria Cheng in London; Lolita C. Baldor in Washington; Sarah DiLorenzo in Dakar, Senegal; Nicolas Garriga and Sylvie Corbet in Paris; and Boubacar Diallo in Conakry, Guinea, contributed to this report.Suggest a correction