Who was the little girl being possessed by Orochimaru during his fight with 3rd Hokage?
I believe you are talking about the girl Orochimaru "became" during the fight with the 3rd Hokage on the roof during the Chunin exams (Ch. 121). I just went back and read that part of the story and I don't believe her identity was ever stated. This is the episode in the anime where he rips his face off and reveals this girl's face:



In other words this is the girl he used as his vessel.