I guess my first question would be, "what kind of doctors." If its only pediatricians telling you that, I would find a pediatric psychiatrist and get that opinion.
There are some other resources that will help. It does sound like the possibility of ADHD is there. The book I recommend a lot is "The ADD/ ADHD Answer book," by Susan Ashley. It covers a ton of things that will help you now and in the future.
Another book I recommend a lot on the child behavior forum is, "SOS Help for Parents," by Lynn Clark. She uses the timorous system as well, but shows in detail how to use it effectively. I have found that many parents who say they use it, really aren't using it correctly or expecting faster results then is possible and then giving up. And its important to do it correctly. How you handle a 3 year old is very different than how you deal with a 5 year old. You also might try posting over on the child behavior forum for ideas on how to work with her. Hope this helps. Best wishes.
Well, it's hard to diagnose her over the internet, but it sounds like she has pica. My cats have this. Also boys accidentally kill baby animals all the time. Try not making a big deal out of everything and maybe it will help calm her down.