They are all alternate reality versions of the same woman, Saturnyne Earth-9, Sat-Yr-9 Earth-794, Ross Earth 616. Although Courtney wasn't originally meant to be the 616 counterpart to them, she was just Brian's ex-girlfriend. Sat-Yr-9 killed and replaced Courtney pretty early in Excalibur and AFAIK has been impersonating her ever since.