After more than seven weeks without a new infection, Congo had been due on Sunday to mark an end to the second-deadliest outbreak of the virus on record, until a case was confirmed on Friday in the eastern city of Beni.
The response to the epidemic, which has killed more than 2,200 people since August 2018, has been hindered by public mistrust and militia attacks that prevented health workers from reaching some hard-hit areas.
All three of the new cases had visited the Horizon Healthcare Centre in Beni, Diallo said.
The girl started showing symptoms only on April 11, meaning she has a good chance of survival and is not thought be very contagious, said Boubacar Diallo, deputy incident manager for the WHO's Ebola response.
"We think we will quickly contain this outbreak," he said.
It's not yet clear how the first case, a 26-year-old electrician, contracted the virus. He had no known contacts with other Ebola patients and was not a survivor of the virus who could have relapsed, the government said on Friday.