i believe star wars was a standalone, tesb the fifth in a nine part story, and rotj the sixth in an hexalogy. i also think that during pre production and before pre production even started for tesb, the idea was to make twelve films, where star wars was the first and tesb the second, and so on.
Lucas’s notes (which I can’t seem to find) for the twelve part saga go Prologue, PT, Interlude, OT, Interlude, ST, meaning ANH would’ve been Episode VI.