Your kingdom has been aided, for as long as anyone remembers, by three great seers. These seers are all-knowing and will truthfully answer one question of any person that seeks them. This question must be addressed at one seer and that seer will answer the question. It is not possible to seek out the seers more than once or to ask a second question of a second seer.
One of the seers has gone mad recently. Instead of answering any question truthfully, he now randomly tells either the truth or a lie. Your king has tasked you with executing the mad seer to prevent him from affecting the sanity of the other seers. Unfortunately, the Seers are an identical triplet (they look and sound identical) and they are always together in the same cave hence figuring out who the mad seer is is problematic in itself. As multiple seers don't serve a purpose anyway, the king has ruled that, if needed, you are allowed to kill one sane seer if it ascertains the death of the mad seer. If you kill both sane seers or fail to kill the mad seer then you will be executed.
You arrive at the seers' cave and go in. Before you stand all three seers. As a person that sought out the seers you are allowed to ask one seer one question.
Which question must you ask and must you kill two seers, or is it possible to kill only the mad seer?
EDIT:: As pointed out in the comments: I forgot to mention its only yes/no questions.