Three teen- agers charged with murder in the death by drowning of a homosexual man who was thrown into a stream pleaded guilty to manslaughter today in a proceeding that drew protests from homosexual rights activists.

James Baines, 15 years old, Shawn I. Mabry, 16, and Daniel Ness, 17, all of Bangor, entered their pleas of admission, the equivalent of guilty pleas in adult cases, at a juvenile hearing before Judge David Cox of District Court.

The three teen-agers had been arrested on murder charges in the drowning of Charles O. Howard, 23, who was assaulted and thrown over the State Street Bridge into the Kenduskeag Stream in downtown Bangor on July 7. He reportedly told his assailants he could not swim.

The hearing was held less than three weeks after Judge Cox had rejected a prosecution motion that sought to have the defendants bound over to Superior Court for trial as adults.