FWIW, the general consensus is that 3 star girls are far more trouble than they are worth, unless you're whaling. They are relatively strong at first (of course) but it is *far* harder to get additional stars for them. You're starting girls could be 6 or 7 star before you finally get one of them to 4 star.

You can buy 3 Naladia and Emily shards every shop reset from the Arena Store (assuming you have 160 arena coins per girl) and 3 Trisha and Elnora shards every shop reset form the league shop (again, 160 league coins each girl)

You get league coins very fast, and Arena coins relatively fast, so you should probably be able to make at 2 purchases from the arena store and 4 from the league store every day.

Which obviously makes Naladia, Trisha, and Elnora the fastest girls to star up.