Step 1: Simplify the expressions.
We are asked to compare \( \frac{x^m}{x^3} \) and \( \frac{m}{x^3} \). Since \( m \) is a multiple of 3, let \( m = 3k \) for some integer \( k \). Thus, \( \frac{x^m}{x^3} = x^{m-3} = x^{3k-3} = x^{3(k-1)} \).
Step 2: Compare the quantities.
Now, we are comparing \( x^{3(k-1)} \) with \( \frac{m}{x^3} = \frac{3k}{x^3} \). Since the relationship involves powers of \( x \) and a coefficient \( m \), we find that for any values of \( k \), the quantities are equal.
Step 3: Conclusion.
Thus, the two quantities are equal.
Final Answer:
\[
\boxed{\text{The correct answer is (3) The two quantities are equal.}}
\]