Prisoner's Dilemma

Three prisoners, A, B and C with apparently good records have applied for parole. The parole board has decided to release two of the three and the prisoners know this. They do not know which two can be released. A warder friend of Prisoner A does know who is to be released. Prisoner A realises that it would be unethical to ask the warder if he, A is to be released but thinks of asking for the name of one prisoner other than himself who is to be released. He thinks that before he asks, his chances of being released are 2/3. he thinks that if the warder says "B will be released" his own chances have now gone down to 1/2 because either A and B or B and C are to be released. And so A decides not to reduce his chances by asking. However A is mistaken in his calculations.

Explain