That would seem obvious, but I do think this is the first one with the potential to decrease each installment. VIII's drop could be softer than ESB's or AOTC's because of the cliffhanger (opposed to ANH) and people generally loving TFA (opposed to TPM), hence tons of interest in what happens next and possible bigger audience retention (also think THG to CF). IX could end up dropping from that because VIII already did better than normal for the second installment, and because SW franchise fatigue setting in by then. None of the other two trilogies had to deal with such a rapid onslaught of SW movies, so we don't know how that affects the trilogy. Poor IX has the disadvantage of potentially being the fifth film in four years, so I don't know that that bodes well for it. I wouldn't be surprised with something like this:
TFA: 1b
VIII: 825m
IX: 775m
Either way, it's amazing this whole trilogy has a fantastic shot at all outgrossing Avatar DOM.