Step 1: Analyze the behavior of the cosine function.
Since the function involves \( \cos 3x \), which has a range of [-1, 1], the denominator of the function will be in the range [1, 3]. Therefore, the range of the function \( f(x) \) will be [1, ∞).
Step 2: Conclusion.
Thus, the range of the function is [1, ∞).
Final Answer:
\[
\boxed{[1, \infty)}
\]