There are safety concerns about leaving ANY child alone with ANY dog at any time. Certainly the larger the dog the greater the risk would be. Do not leave your child unsupervised with the dog even if you're certain that "nothing will happen" because any animal can have a bad day.
Young children should never be left unsupervised with any dog. Dogs can injure a child simply while being playful such as by jumping or knocking a child over. There is also always a possibility that a dog may react badly to a childs' attention or physical contact. Always be cautious.
You should never leave a child unsupervised with a dog, regardless of the bread. German Shepherds are natural hearding and guarding dogs. A child becomes more at rish for being hurt when he or she is screaming or making noise.
Yes, I would be quite concerned if the dog and child will be left alone together. All dogs are unpredictable and so are three year olds. I would never leave a young child alone with any dog, regardless of breed.