To determine the number of fish \( f \) in the bucket before Lester feeds the dolphins, we can start by setting up an equation based on the information given.
Lester divides the fish evenly among 3 dolphins, and each dolphin gets 6 fish. Therefore, the total number of fish in the bucket can be represented by the equation:
\[ f = 3 \times 6 \]
Now, let's simplify this equation:
\[ f = 18 \]
Thus, the number of fish in the bucket before Lester feeds the dolphins is \( \boxed{18} \).