What if I ask A if they feel bad when they lie?...if they can't answer (because they never lie) then they must be the truth teller.
If they answer yes or no then ask B the same question... again if they can't answer they are the truth teller...
If they answer yes or no then C is the truth teller...
SO NOW ASK THE TRUTH TELLER IF (A) IS THE LIER...HIS ANSWER SHOULD SOLVE THE ISSUE !!!