R. Kelly Case: Three-Way in Play

The platinum pisser may be the one getting screwed this time.

A woman is set to testify at R. Kelly's upcoming child pornography trial, revealing that she had a threesome with Kelly and the allegedly underage girl shown in the infamous video.

Sources tell the Chicago Sun-Times that the new witness not only will testify that she was underage when she had the threesome with Kelly -- but also that the other girl involved is the exact same person who is now identified as the alleged victim.

Prosecutors claim to have identified the girl in the videotape -- who is now in her 20s. She denies it's her on the tape. She will take the stand in the trial as well.

Jury selection for the trial begins Friday. Both sides declined to comment, citing a gag order in the case.