I don't agree with any of these options, but I proposed the question back in 2014, so I hope I can shed some light. The difference between non-cognitivism and error theory is that the error theory supposes that people attempt to describe some feature of the world when they make moral statements, and that feature doesn't exist, while non-cognitivism holds that moral statements only express emotional attitudes ("Yay for X!") or commands ("Don't X!"), which can neither be true nor false. The difference between error theory and subjectivism is that subjectivists believe that some moral statements are true, but that they are made true by something mind-dependent (but what counts as mind-dependent turns out to be quite complicated).