$$
\begin{array}{c}
\text { 3. Given } x+y+z=3 a \quad(a \neq 0) \text {, then } \\
\frac{(x-a)(y-a)+(y-a)(z-a)+(z-a)(x-a)}{(x-a)^{2}+(y-a)^{2}+(z-a)^{2}}
\end{array}
$$

The value is $\qquad$
3. $-\frac{1}{2}$,