How do I find a function $f(3n) = 3n$ such that it is different from the identity function? This is my first time posting. I'm sorry if I'm neglecting some good etiquette practices; I tried to read everything that's been sent my way, but I probably missed something anyway. Also, English is not my first language, so I'm relying on Google to translate math-specific terminology. If something isn't clear, please let me know!
I'm a Computer Science student at University, and I've been requested to find a function $f: \Bbb{N}\to\Bbb{N}$ such that $\forall n \in \Bbb{N}$,  $f(3n) = 3n \land f\neq \mathrm{id}_\Bbb {N}$. I absolutely cannot find a solution, as $f(x) = x$ (and, as such, $f(3n) = 3n$ too)  literally is the definition of identity function as far as I know... Am I missing something? Thanks in advance.
EDIT: Thanks a lot everyone!

You want $f|_{3\mathbb N}$ ($f$ restricted to multiples of $3$) to be the identity function
but $f$ not to be the identity function.
There are many ways to accomplish this; others have given some.  Here are two alternatives:
(a)  $f(n)=n$ for $n\ne1$ and $f(1)=2$
(b)  $f(3n)=3n$, $f(3n+1)=3n+2$, and $f(3n+2)=3n+1$.
