Three children who received blood transfusions at Childrens Hospital of Orange County have been infected with the AIDS virus, the hospital's chief of infectious diseases said. One of the three has developed early symptoms of the fatal disease, Dr. David J. Lang said. The three unnamed youngsters were identified after a Red Cross investigation backtracked the tainted blood from donors who were subsequently found to have AIDS. Lang made the disclosure while requesting that parents of all children who received transfusions at the city of Orange facility from 1978 to March, 1985, contact the hospital to arrange for the children to be tested for the AIDS virus. Parents who are unsure whether their children received blood products while in the hospital are encouraged call 714-532-8686 to find out, Lang said.