I'll try to make it more like the Monty Hall problem so it can be better understood...
Ok... say you were to choose which prisoner was to ask the warden. You have a 2/3 chance of choosing a prisoner that is to be released, correct? And every time you choose a prisoner that is to be released, the warden will reveal the other prisoner that is to be released to them, and the person left over will therefore be the one sentenced to death.
So if the situation is like that 2/3 of the time when you choose the prisoner to ask, then the probability of the third person to be sentenced to death would be the same, 2/3.
But like Steph mentioned from the wiki... if A had asked "Will B be released?", the warden would've said yes, and A and C's probability would be the same, 1/2.