To write the equation \( x - y = 3 \) in function notation with \( x \) as the independent variable, we need to solve for \( y \).
Starting with the given equation:
\[ x - y = 3 \]
We can rearrange it to solve for \( y \):
\[ -y = 3 - x \]
Now, multiply both sides by -1:
\[ y = x - 3 \]
Now we can express this in function notation, where \( f(x) = y \):
\[ f(x) = x - 3 \]
Thus, the correct answer is:
\( f(x) = x - 3 \).