It's hard to say exactly without reading it. But usually, there could be new plot developments or character interactions.
In chapter 3, there might be a twist in the story, maybe a new challenge for the characters or some revelations. It all depends on the author's imagination and the direction of the plot.
Well, chapter 3 of a certain scientific accelerator manga could introduce new elements like a powerful enemy or a key discovery. It could also deepen the relationships between the characters or move the main storyline forward in an unexpected way.