3. Given that $f(x)$ is an even function, for any $x \in \mathbf{R}^{+}$, we have $f(2+x)=-2 f(2-x)$, and $f(-1)=4, f(-3)$ equals $\qquad$ .
3. -8 .

Let $x=1$, then $f(3)=-2 f(1)$. Since $f(x)$ is an even function, then $f(-3)=f(3)=-2 f(1)=$ $-2 f(-1)=-8$