I agree the first is the best. Wonderful headshot. However, the third is absolutely adorable! She has such a great, natural, and real smile. In this, imho, the slouch doesn't matter. Great work!
Hmmm... one or two other thoughts...
As per Peter Hurley (top head shooter in NY and LA) however, the hair is best in the second photo. You might not have her (or future female subjects) wrap her hair around her neck. The neck should be open, and the jawline emphasized.
Also, in general (not just with Peter Hurley), often the stronger view is of the side where the person, male or female, parts their hair. That's not always the case, but often it is.
Regardless, the photos are very nice, and she's extremely pretty. Very nice work!