Winston is still thinking in the majority of Book 3. He has the ability to wonder, to feel pain, to think about pain, and to question. Winston is sane according to our understanding of sanity. O'Brien intends to make Winston sane according to Party standards, which means that Winston will believe what he is told, even if his eyes tell him differently. They are trying to torture the thought out of him and Winston just won't break.
Shall I tell you why we have brought you here? To cure you! To make you sane! Will you understand, Winston, that no one whom we bring to this place ever leaves our hands uncured? We are not interested in the stupid crimes you have committed. The Party is not interested in the overt act: the thought is all we care about. We do not merely destroy our enemies; we change them.
O'Brien was furious in this moment in Book 3, chapter 2. Winston would not have the concept of truth changed in him at this point. Here is how the Party finds that Winston has failed to make progress.