Also...how sure was Cercei that all three kids were Jamie's? If she was sleeping with both Jamie and Robert surely there is a chance that one or more of them was Robert's.
Did they know enough about fertility and ovulation for her to time events to make it more likely that they would be Jamie's kids?
In the books, she's a point of view character and IIRC, in one chapter she recollects how she used to finish Robert with her hand or mouth and not let him ejaculate inside her, as she didn't want to bear his children. He was generally too drunk to catch on to what she was doing.