I would take her to see a Naturopathic Doctor who can test for food allergies and food intolerances. If she is having diarrhea that much, her intestines are not only messed up and inflammed, but something she is eating (or drinking) doesn't like her body. I would want to find out what that is and avoid it. It will take a while for her intestines/colon to heal if the diarrhea has been ongoing for three months.
Some people who are allergic to the proteins (casein or whey) in cow's milk suffer from diarrhea (my husband); others have constipation (me and my daughter). Is your daughter breast fed or taking formula or drinking cow's milk? Did the Ped make any mention about trying to eliminate cow's milk/products from her diet?
Just a note: the tests the allopathic doctors run for food issues are different from the ones a Naturopathic Doctor will run. For what it's worth--I tend to put my faith in alternative healthcare doctors. I got seriously ill in 2008 and had absolutely no help from the allopathic doctors I was seeing. I finally decided it was my body and I wanted to be well, so I went and started checking out the alternative medicine side of things. I have never been more thankful in my life that I "stepped outside the boundary of what's considered 'medicine'" and embraced alternative healthcare. I found answers to my health problems and dis-eases there.
I had a Biopsy of both my large and small intestine done, to make sure I didn't have Celiac Disease (I have severe gluten intolerance). It's not a nice procedure, but if the doctor knows what he's doing/what to look for, it is a good procedure. Before I'd go that route, however, I would try doctoring with a Naturopath. Just my opinion.
And anytime diarrhea has been going on that long, there is lots of inflammation inside the body. Until you get to the ROOT CAUSE of what's triggering the severe diarrhea and TAKE AWAY the irritant that's causing the diarrhea, her colon/intestines won't be able to heal and get better. Another thing: with her colon/intestines inflammed like they are, it's a sure bet that her body isn't able to pull out all the nutrients it needs from the food(s) she eats. She might be malabsorbed. I had severe malabsorption by the time I finally went to see a naturopathic doctor at the age of 40 years because of all the years I'd lived with severe food allergies and food intolerances.