Girls that start at three stars are not viable unless you're a whale. Anyone else won't be able to star them up very far. A girl that started at one star and has been ranked up to seven is usually going to be a lot stronger than one that started at three stars and has only managed to get to four.

Elnora and Chharodi are probably the strongest of the one-star girls for which people have had a realistic chance to get them to seven stars by now without going full whale. (The others that you could readily have at seven stars by now are Meryl, Emily, Trisha, and Naladia.) They're both good for arena and tower of trials, though less good for raids because it takes so long to accumulate enough SP to use their skills.

For raiding, Grace and Seleia are probably the strongest characters. They start at two stars, so it's possible to rank them up eventually without needing to be a whale. They can crank out damage pretty fast without having to wait for skills. They're both good for other types of content, too, but nowhere near as dominant as in raiding.

Of the three star girls, Claire and Khalis are mediocre for PVP. Isabel is better, but hardly dominant, and you'll have to get her all the way to seven stars in order to really be viable. That's not going to happen unless you're a whale. The newer three star girls haven't been out for long enough for me to see them used much.