logical deduction - Who stole the flying car in Hogwarts?

During a trial in a Hogwarts court, Ron claimed that the Flying Ford Anglia car was stolen by Hermione.

Then Hermione and Harry gave testimonies which, for some reason, were not recorded. Later on in the trial it was found that the car was stolen by one of these three defendants, and moreover, only the guilty one gave true testimony.

Who stole the car?

Answer

Only the guilty one gave true testimony.

There are 2 options for being guilty and giving true testimony:

1. 
   

   
   

   Declaring yourself guilty

   
   

   
   


2. 
   

   
   

   Declaring someone else not guilty

   
   

   
   

Ron declares someone guilty different than himself (1), which means

he's lying.

If Ron lies, he can't be the guilty one, so he's innocent.

If Ron lies, what he says is false. So Hermione is innocent as well.

If Ron and Hermione are innocent, Harry is guilty (and he'll admit it).