Let $ f$ be a function satisfying $ f(xy) \equal{} f(x)/y$ for all positive real numbers $ x$ and $ y$. If $ f(500) \equal{} 3$, what is the value of $ f(600)$?

$ \textbf{(A)} \ 1 \qquad \textbf{(B)} \ 2 \qquad \textbf{(C)} \ \displaystyle \frac {5}{2} \qquad \textbf{(D)} \ 3 \qquad \textbf{(E)} \ \displaystyle \frac {18}{5}$
1. Given the functional equation \( f(xy) = \frac{f(x)}{y} \) for all positive real numbers \( x \) and \( y \), we need to find \( f(600) \) given that \( f(500) = 3 \).

2. First, let's use the given information \( f(500) = 3 \). We can rewrite \( 500 \) as \( 100 \times 5 \). Using the functional equation:
   \[
   f(500) = f(100 \times 5) = \frac{f(100)}{5}
   \]
   Given \( f(500) = 3 \), we have:
   \[
   3 = \frac{f(100)}{5}
   \]
   Solving for \( f(100) \):
   \[
   f(100) = 3 \times 5 = 15
   \]

3. Now, we need to find \( f(600) \). We can rewrite \( 600 \) as \( 100 \times 6 \). Using the functional equation again:
   \[
   f(600) = f(100 \times 6) = \frac{f(100)}{6}
   \]
   We already found \( f(100) = 15 \), so:
   \[
   f(600) = \frac{15}{6} = \frac{5}{2}
   \]

4. Therefore, the value of \( f(600) \) is:
   \[
   \boxed{\frac{5}{2}}
   \]