Step 1: $\backslash(@(A,B), 2)$ = product of $@(A,B)$ and 2 = $A+B$.
Step 2: $@(\ A+B, \ C)$ = average of $A+B$ and $C$ = $\dfrac{(A+B) + C}{2} = \dfrac{A+B+C}{2}$.
Step 3: To get average of $A,B,C$, divide sum by 3:
Average = $\dfrac{A+B+C}{3}$, so divide the sum $(A+B+C)$ by 3:
\[
x(@(\backslash(@(A,B), 2), C), 3)
\]
Thus (3) is correct.
\[
\boxed{x(@(\backslash(@(A,B), 2), C), 3)}
\]