​Can someone explain this to me?

The $\Delta$symbol means take the difference between successive values of ...

So $\Delta f(x)$means take the difference between successive values of f(x): namely 3 here, and $\Delta x$means take the difference between successive values of x, or 1 here.

So $\frac{\Delta f(x)}{\Delta x}=\frac{3}{1}\rightarrow3$