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:

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"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.