A "three" is a little thin. You can try upping her food a bit and feeding her several times per day. If she's getting three total cups per day, give her 3 1/2 to 4 cups, but space it out over the day.
You can also add in fresh veggies and fruits, but do so slowly so she doesn't get diarrhea. Try potatoes and breads, too, again in small quantities to avoid stomach upset.
Your best bet is to feed her a better-quality food. Talk to your parents about switching her food to something more nutritious. If they're willing to do that, look at your local feed stores and find out what they offer, then tell us and we can help you pick the best food.
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