Let, A=[aii]3×3
tr(AAT)=3
a112+a122+a132+a212+.......+a332=3
Possible cases
\begin{matrix}0,0,0,0,0,0,1,1,1 & \rightarrow 1 \\ 0,0,0,0,0,0,-1,-1,-1 & \rightarrow 1 \\ 0,0,0,0,0,0,1,1,-1 & \rightarrow 3 \\ 0,0,0,0,0,0,-1,1,-1 & \rightarrow 3\end{matrix}}{}^{9}{C}_{6}\times 8=84\times 8=672