The Pope's statement during the news conference aboard his flight from Israel back to Italy came after he was asked what he would do if a bishop did not observe Church norms regarding a moral, and often legal, obligation to report allegations of sexual abuse against a member of the clergy.

While condemning the abuse of children as "an ugly crime" and affirming a policy of "zero tolerance" for abusers, Pope Francis did not clarify whether the three bishops he mentioned were under investigation for their handling of abuse allegations or because they themselves were accused of abuse.

Previously, the Vatican had acknowledged formally investigations against two bishops:

-- In April the Congregation for Bishops sent Auxiliary Bishop Charles Scicluna of Malta to Scotland to collect testimony in a case against Cardinal Keith O'Brien, the former archbishop of St. Andrews and Edinburgh, who resigned in 2013 after admitting to sexual misconduct.

-- In early May testimony before a UN committee, Archbishop Silvano Tomasi confirmed earlier Vatican statements that Archbishop Jozef Wesolowski, the former nuncio to the Dominican Republic, is the subject of a canonical investigation by the Congregation for the Doctrine of the Faith as well as a criminal investigation by the Vatican police and court. Wesolowski was removed from his position last August after he was accused of paying for sex with boys in the Dominican Republic.

As for the third bishop, the bishops' conference of Chile published a statement in early February saying that Bishop Cristian Contreras Molina of San Felipe had asked the Vatican to open an investigation into "serious allegations" made against him. Chilean media had reported that the doctrinal congregation sent investigators to the diocese to study allegations involving the sexual abuse of minors.

Although victims' advocates and others have called for Vatican action against Bishop Robert W. Finn of Kansas City-St. Joseph, Mo., there is no indication that a Vatican investigation is under way. The bishop was convicted by a local circuit court in 2012 of one count of failing to report suspected child abuse.