that's correct for tifa. you have to do all the mission in ch3, get the choice in the dialog by going back to the inn, then finish the chapter to save the choice. the most recent way you did ch3 will affect the next ch9 dress selection for tifa.

for aerith, it's the number of missions you do in ch8 (0, 3, 6 for dress quality respectively) and you still have to finish the chapter.

for cloud, you have two dresses depending on who will be your "mission provider" (either choco Sam or mdme what's her name), and you have to do them all, or do no missions and just return directly for aerith/jonny event.

it did get annoying to try and get all 9, esp cause I forgot what dress tifa had already had... had to redo ch3 3 times. if you don't trigger the decision dialog by doing all the ch3 missions she will have the mature dress, and I initially picked that one like a sucker.