"Three monks were subjected to a criminal assault inside our Coptic monastery," the spokesman of the Coptic Orthodox Church said in statement posted on Facebook, without providing other details.
One of the three monks killed was a representative of the Coptic Diocese of South Africa.
An internal investigation has been launched, the statement added, and the Egyptian ambassador to Johannesburg, where the monastery is based, has been informed.
(Reporting by Hatem Maher and Adam MakaryEditing by Chris Reese and Leslie Adler)