A film should work within itself regardless of what Episode it is or something. ROTS is quite competent at establishing their relationship and what is it that falls apart - ditto for Anakin!s relationship to Padmé. Not a surprise it was reviewed more positively than all other Lucas SW films upon release - with the exception of the original.
Well, establishing what “it is” and the audience actually experiencing and feeling it…are two different things. And I wouldn’t say Episode 3 does the job well considering Anakin and Obiwan separate on their own missions for a majority of the film. Obi-Wan isn’t even present during the whole “turn” and major plays that went down on Coruscant. Obi-Wan basically fights him just because he’s on an assigned Jedi mission from Yoda to take out the “other half” of the Sith duo situation. I think it’s a stretch to say ROTS stands on its own. And these movies KNOW they are parts of a trilogy. Return of the Jedi doesn’t really stand on its own.