First, we need to calculate \( f(-2) \) using the function given:
\[ f(x) = 3x - 3 \]
Substituting \(-2\) for \(x\):
\[ f(-2) = 3(-2) - 3 \]
Calculating this step by step:
\[ f(-2) = -6 - 3 = -9 \]
Now, we need to find \( f(-2) - 12 \):
\[ f(-2) - 12 = -9 - 12 \]
Calculating that gives:
\[ -9 - 12 = -21 \]
Thus, the final answer is:
\[ \boxed{-21} \]