Red K does NOT split Supes in SM3.
It's a synthetic Green K that Richard Pryor had made because he couldn't figure out what the last ingredient in Kryptonit was... so he substituted TAR as an ingredient (he was reading from a pack of smokes at the time). Instead of killing Superman like they thought it would, the green K slowly warped him into a bad guy, eventually splitting him in two.
The movie overall was pretty crappy, but the fight between Clark and bad Superman is still one of my favorite comic movie moments.