3. Positive real numbers $u, v, w$ are not equal to 1. If $\log _{u} v w+\log _{v} w=5, \log _{v} u+\log _{w} v=3$, then the value of $\log _{w} u$ is $\qquad$
$$
\Rightarrow 4 b=5 \Rightarrow b=\frac{5}{4} \Rightarrow \log _{w} u=\frac{1}{b}=\frac{4}{5} .
$$