> How do you know what her ability in English would be if she'd not had the other 2 languages.
Not OP. I can speak 3 Indian languages and English. My English is better than native English speakers, I know that because I got above average points in IELTS.
Yes when you are learning as a kid things will be little show at first but most kids (sample size of my Indian friends who are trilinguals as well) will be fine after some years.