∵A is orthogonal matrix
$\begin{aligned}
& \therefore A^T=A^{-1} \
& \Rightarrow A^2=A^{-1} \left(\because \mathrm{A}^2=\mathrm{A}^{\mathrm{T}}\right) \
& \Rightarrow A^3=I \
& \text { let } B=(A+I)^3+(A-I)^3-6 A \
& =2\left(A^3+3 A\right)-6 A \
& =2 A^3 \
& B=2 I=\left[\begin{array}{lll}
2 & 0 & 0 \
0 & 2 & 0 \
0 & 0 & 2
\end{array}\right]
\end{aligned}$
Now sum of diagonal elements =2+2+2=6