DUNCAN, Oklahoma (REUTERS) - One of three Oklahoma teenagers charged with fatally shooting an Australian student athlete who was jogging in a small town last August indicated in a court hearing on Tuesday he would testify against the other two in exchange for a reduced charge.

James Edwards Jr, Chancey Allen Luna and Michael Dewayne Jones were charged as adults with first-degree murder in the shooting of Christopher Lane, a Melbourne resident who was attending university in Oklahoma on a baseball scholarship.

The teens are accused of tracking Lane, who police said was shot in the back on Aug 16 while jogging in Duncan, Oklahoma, a town about 130km south of Oklahoma City.

Lane, 23, was a student at East Central University in Ada, Oklahoma.

At an evidentiary hearing on Tuesday, Edwards said he was in the front passenger seat and rolling a marijuana joint while Jones was driving the car and Luna was in the backseat.

"I heard a gunshot," he said. "I heard it from the right side of the car." Edwards said he looked in the mirror and saw Lane lying on his stomach, and that Luna - with a "shocked and cold expression" - said he thought there were supposed to be blanks in the gun.

If he continues to testify, prosecutors said, Edwards would face a lesser charge of accessory to murder.

According to police, Jones had said the teens had decided to kill someone "for the fun of it".

Police have said Jones told them they saw Lane jogging, decided to target him and then followed him in their vehicle before shooting him.

If convicted of first-degree murder, the teens face a possible sentence of life in prison without parole. Oklahoma law allows a death sentence for first-degree murder, but people convicted of capital crimes committed before age 18 cannot be sentenced to death in the United States.