The three patients include a 10-year-old boy from Paynesville, a suburb of the capital Monrovia.
All people with the symptom have been isolated, WHO spokeswoman Margaret Harris told the BBC.
Liberia has seen more than 10,000 Ebola cases and more than 4,000 deaths.
The country’s Health Minister Bernice Dahn said six of the boy’s relatives and other high-risk contacts have been taken to an Ebola Treatment Unit in Paynesville.
“The hospital is currently decontaminating the unit. All of the healthcare workers who came in contact with the patient have been notified,” she said.
On Thursday night, before the new case emerged, President Ellen Johnson Sirleaf delivered a speech praising “the resilience” of Liberians in overcoming the epidemic.
The country was first declared Ebola-free on 9 May, but new cases emerged in June resulting in two deaths. It was declared free of the virus again on 3 September.
Liberia recorded its first Ebola case in March last year and analysts believe the latest cases are a serious set-back for the country.
The new cases in Liberia was announced just days after Guinea, where the epidemic started, said it had no more Ebola cases.
If no more cases are detected for 42 days, it would be declared free of the virus.
Sierra Leone was declared free of Ebola on 7 November.
More than 11,000 people have died of the disease since December 2013, the vast majority of them in Liberia, Guinea and Sierra Leone.