Remember, in season one when Five stole Three’s bullets? Remember how he disliked her crawling in the vents? Remember what old Marcus Boone wanted to do to her?
Now Three can’t even look at her being sad. Look how proud he is of himself just because he could make her feel a little better.
And she hid in his childhood memories - they were her safe place when she was lost.
At the beginning this seemed to be the most unlikely friendship but look at them now. She’s his little sister and he doesn’t even want his big gun back.