What could be the three main positive changes in charcters in The Outsiders?
You have identified an important theme. As the novel progresses, we see definite changes in the characters and how they mature and develop. I would want to argue that the changes are most obvious in Ponyboy, Johnny and Darry.
Ponyboy and Johnny we can link together by considering Johnny's final letter that he wrote to Ponyboy which is read by him in the last chapter. A lot of the learning and development that goes on in them is as a result of the experiences that they have shared together, but also it is based on reflecting upon the Robert Frost poem, "Nothing Gold Can Stay," that is so important to the novel. The key part of this letter in terms of Johnny is when he says:
The doctor came in a while ago but I knew anyway. I keep getting tireder and tireder. Listen, I don't mind dying now. It's worth it. It's worth saving those kids. Their lives are worth more than mine, they have more to live for.
We see here that Johnny comes ot accept his death and, tragically, to welcome it, looking upon death as a release from a profoundly unhappy life.
However, although his ending is sad, it is clear that he offers hope and inspiration to Ponyboy:
You still have a lot of time to make yourself be what you want. There's still lots of good in the world.
He tells Ponyboy, and from this Ponyboy receives the inspiration to write about his story to produce a change, that, unlike him, Ponyboy is still able to change himself and give himself hope and a future. Ponyboy does not have to feel condemned to living as a greaser for the rest of his life.
The change in Darry comes after the fight that he and Ponyboy have and the way that Sodapops manages to heal the breach between them when he opens up about how he is feeling:
"We're all we've got left. We ought to be able to stick together against anything. If you don't have anything, you end up like Dallas... and I don't mean dead either, I mean like he was before. And that is worse than anything. Please... don't fight anymore."
Thus Darry and Ponyboy change in terms of how they relate to each other because of their love for Sodapops and their desire not to make him cry.