The beauty of this shop is that part 3 is widely available and if you get turned away by 1 girl there will be countless others that will offer it.
I haven’t seen many of the new batch of girls but my personal experience with the girls I’ve seen is that some of the girls allow part 3 to happen as a natural progression in the heat of the moment. For the other times I’ve just asked (once) and the girls were happy to do it.