Let $S$ be the set of all $p$-adic integers $\mathbb{Z}_p$. What is the $3$-adic absolute value $|18|_3$?

Note: The $p$-adic valuation $v_p(n)$ is the exponent of $p$ in the prime factorization of $n$. The $p$-adic absolute value is defined as $|n|_p = p^{-v_p(n)}$.