A book has 3 chapters. The first chapter is 35 pages long the second chapter is 18 pages long and the third chapter is 3 pages long. How many more pages does the second chapter have than the third chapter?
Let's denote the number of pages in the first chapter as $x$, the number of pages in the second chapter as $y$, and the number of pages in the third chapter as $z$. 

According to the problem statement, we have the following system of equations:
$$
\begin{cases}
x=35,\\
y=18,\\
z=3,\\
y-z=15.
\end{cases}
$$

This system has a unique solution: $x=35$, $y=18$, $z=3$. 

Therefore, the second chapter has 15 more pages than the third chapter.