^Supergirl would be redundant between Superman and Wonderwoman.
(then again, an NC-17 movie of Supergirl between Superman and Wonderwoman would be
Supergirl doesn't have quite the same powers as wonder woman, she doesn't need an invisible plane, and she don't need bracelets to deflect bullets, they just bounce off her skin instead. I never heard of Wonder Woman having x-ray vision or superhearing.
Also Supergirl is a much younger superhero than wonder woman who is thousands of years old. Supergirl's youth and inexperience make for a lot of interesting adventures with her getting into troubles that Wonder Woman would know better than to get into.