1 Haven't done it in a while, but iirc, the first time you buy the dress it's on the way to or from helping Brenda (I think) somewhere around midday. You'll have 2 or 3 options, and have to pick the right one for Ann; also, you'll need to have enough money for it.
3 The main interaction with Rhonda that lower her inhibitions are in the morning when she's cleaning the living room, chat her up, one or both of the dialogue trees lower them a bit; they re-raise by a small amount every day, so you'll need to go for every interaction you have together in a day to lower it to a point where it can be kept low when progressing other routes.
If you're not meaning Rhonda, I don't know who you're meaning.