This sounds much worse than a food aggression issue. It is possible that the 3 yr old considers this her turf and does not accept the other dog being there at all.
You should read https://pethelpful.com/dogs/dog-to-dog-aggression
Try all of the things that i recommend in that article. Do not give up on your dogs until you have tried those things.
Sometimes two females or two males never get along. You have to rehome one of the dogs. When you adopted the second dog, did you discuss this? I know a pit mix is going to have a hard time finding a new home, but keep in mind that this is something you might need to consider if things do not work out.