A defrocked Philadelphia priest pleaded guilty Thursday to sexually abusing an altar boy in a church sacristy, days before he was to go on trial with two other priests in a landmark child sex-abuse case.

Edward Avery, 69, known for his moonlighting work as a disc jockey, pleaded guilty to involuntary deviate sexual intercourse and conspiracy to endanger the welfare of a child. He was immediately sentenced to 2 ½ to five years in prison.

“In the end, every human being proceeds on this earth as a flawed human being. Father Avery has made some horrible mistakes in his life,’’ defense lawyer John P. Donohue said at the brief hearing.

The charges stem from Avery’s abuse of an altar boy at St. Jerome’s Parish in northeast Philadelphia in 1999, when Avery was 57 and the boy 10.

Two other priests are still set to go on trial Monday. They include Monsignor William Lynn, the first U.S. church official ever charged with endangering children for allegedly failing to oust accused predators from the priesthood or report them to police.

Avery was at St. Jerome’s despite a credible 1992 complaint that led him to undergo psychological testing at an archdiocesan-run psychiatric hospital, according to a 2005 grand jury report. He was pulled from his parish, put on a so-called “health leave’’ and then reassigned in 1993, the report said.

Avery’s accuser told authorities in recent years that he had been raped by two priests and his sixth-grade teacher at St. Jerome’s.

A second grand jury report on priest abuse issued last year charged all three of them with sexual assault and alleged two attacks by Avery.