They were three absolutely different players. But it was originally written for Knushevitsky. He added some passages to it. There were some bad rumors which said that Rostropovich simply knocked Knushevitsky out of his place. Knushevitsky was a much gentler person, that's for sure, but he was also unfortunately quite a drunkard.
People keep forgetting that.
In later years, when he was in his mid-60's he could barely play because he drank so much. It was such a bad habit that with or without Rostropovich he simply was not recuperating much. When I was in the Moscow Conservatory, I could have basically skipped everything except my cello lessons. I could have easily skipped anything. I could skip orchestra all together, Party related things, classes about the history of communism.