The heartbroken girlfriend of slain Australian baseballer Chris Lane says she doesn't care what happens to the "evil people" who gunned him down in the US.

Mr Lane, 22, had been in the US on a sports scholarship with East Central University (ECU) as the team's catcher.

He was going for a jog along a road in the Oklahoma town of Duncan on Friday when he was shot in the back.

Three boys - aged 15, 16 and 17 - were due to appear in a local court over the shooting on Monday afternoon (about 4.30am Tuesday AEST), but it was pushed back a day.

Speaking from Duncan, Mr Lane's girlfriend, Sarah Harper, said she wasn't thinking about the killers at the moment.

"I don't really care what happens to them," she told the Nine Network.

"I feel that if they don't get what they deserve now in the present they will eternally.

"They're evil people and they'll be taken care of eventually."

Witnesses saw a wounded Mr Lane stumble across the road before falling to his knees and collapsing.

Mr Lane's father, Peter, has described his son's death as "senseless".

Ms Harper said her life with Mr Lane had been "amazing".

"He was an amazing person, an amazing friend," she said.

"He didn't deserve any of this. It's heartbreaking that it was such a random choice that those guys made."

If convicted of first degree murder, the accused boys face a maximum sentence of life in prison.