To find \( \frac{1}{6^3} \), we first calculate \( 6^3 \):
\[ 6^3 = 6 \times 6 \times 6 = 216 \]
Therefore, we have:
\[ \frac{1}{6^3} = \frac{1}{216} \]
Now let's compare \( \frac{1}{216} \) with the provided options:
- \( \frac{3}{1296} \)
- \( \frac{1}{216} \)
- \( \frac{3}{18} \)
- \( \frac{1}{9} \)
The second option, \( \frac{1}{216} \), matches our result.
Thus, the answer is:
\[ \frac{1}{216} \]