You'd be best served by reviewing the CRLs for each one.
Off the top of my head, between ANH and ESB, I believe the differences are holster attachment, hand guards, and frown painting. There may be more that I just don't realize, as my research has all been geared towards ANH Centurion standards.
ROTJ has no holster, ESB hand guards, similar painting on the helmet (although rank stripes are increased), rubber trim around the body pieces, different arrangement of the ab buttons, no buttons on the belt, and all white boots.
As I said, I may be omitting a lot, but off the top of my head, that's the bulk of it - outside of differences in the weaponry.