Answer
Shrink the graph of $y=f(x)$ horizontally by the factor of $3$.
Work Step by Step
What 'happens' to the function $f(x)$ at $x$ the same will 'happen' to the function $f(3x)$ at $x/3$, which means that to get the graph of $y=f(3x)$ we have to shrink the graph of $y=f(x)$ by the factor of $3$ horizontaly.
