There are several examples of foreshadowing on the last pages of Chapter 3 of The Outsiders. One comes when Ponyboy is slapped by Darry, and he tells Johnny to "Come on... we're running away." Although Pony's original intent was to cool off and return home, events force the boys to leave town and hide out at the abandoned church on Jay Mountain. Later, Pony tells Johnny that Darry "couldn't stop me from living in my own house." Of course, the murder in the park does prevent Pony from returning home. The final sentences of the chapter are the most ominous, however.
Things gotta get better, I figured. They couldn't get worse. I was wrong.
Things turned drastically worse moments later when Pony and Johnny are attacked by a group of Socs in the park, resulting in the death of Bob Sheldon, and Pony and Johnny become fugitives on the lamb.
The quick answer to this question is Ponyboy’s statement
Things gotta get better, I figured. They couldn’t get worse. I was wrong.
The words “I was wrong” make the reader think, “oh boy, what’s gonna happen next.” So, delving further into what Hinton has done in chapter 3 helps the reader to see that she has set the stage with what Ponyboy considered the worst thing that could happen — he gets home way past his curfew and argues with Darry. After Darry slaps Ponyboy, he realizes, for certain, Darry didn’t want him around. Ponyboy goes back to Johnny and says he wants to run away. Johnny agrees, which leaves the boys virtually homeless. Things couldn't get any worse than that, or could they? Ponyboy (and Hinton’s readers) find out in chapter 4, not just how much but, how fast things could go from bad to worse.
Hinton had set the stage for this worse case scenario, the fight, when the greasers saw the Socs in the blue Mustang. At that point, Ponyboy realizes the Socs in the blue Mustang are the ones who had attacked Johnny previously, a fight is threatened but thwarted by Cherry, one of the girls Ponyboy and Two-Bit are with. The girls just happen to be the girlfriends of the Socs in the blue Mustang so their choice to go with the Socs instead of staying with the greasers postpones the inevitable fight — until 2:30 a.m., in the park. What happens next is far worse that anything Ponyboy could have imagined. Running away becomes more than running away from home. It's running away from a murder.
At the end of the Chapter, Ponyboy tells Johnny that if he walked to the park and back, he might be cooled off enough to go home. He thought that things ought to be better but at the last sentence Pony said that he was wrong.