How does Winston change in part 3 of 1984?
Winston changes in part 3 through various means. Most obviously, he betrays Julia. It took him a little while to be broken, but he does finally turn on her. Previously, when speaking with O'Brien in Book 2, both Julia and Winston were asked if they would sacrifice each other for the sake of the cause. They said no, they would not betray each other.
Similarly, he has been able to maintain in his mind what he knew as absolute truth for the first two books and the majority of the third book. By the end of the third book, he releases himself completely allowing the torture to totally change him. His mind took on a revised outlook:
He pushed the picture out of his mind. It was a false memory. He was troubled by false memories occasionally. They did not matter so long as one knew them for what they were. Some things had happened, others had not happened.
When he experienced memories, or saw truth playing out in front of him, he now took them for a different value: falsehood. This is complete brain-washing. He allowed it to happen to him. That is why the book closes with the words:
He loved Big Brother.