I agree with clover, try more carbs and less protein before practice. Pretzels & goldfish are a staple in my gymnasts diet, also Cheezits. We focus more heavily on protein at meals after practice or earlier in the morning. It takes a ton of food and is something we put a great deal of thought into. Make sure she is getting enough carbs, protein, etc throughout the day and night. Sometimes we also have to listen to what she wants. There are days when she will eat a ton of rice, salad, goldfish, apples etc. and not as much protein. And there are other days when it seems all she wants is protein. We try to listen to her wants (assuming her body is craving what it needs) as much as we can since she is 10. Depending on how tired and weak your DD is, you might consider a trip to the doctor. 3 hours is not an extremely long practice and most kids should be able to make it with out refueling or getting weak and tired.
How old is she and how many days a week is she practicing?