J
I know right? Okay so the three kids think it's Snape that's trying to get into the door. And they see that his leg is bitten later on. But what happened was that Quirrell was the one who let the troll inside so that he and Voldemort (who is attached to the back of his head) could go get the stone. Everyone would be too busy focusing on the troll to pay attention to the door. Snape was just trying to stop him.
Fluffy.