3. Given $x \in \mathbf{R}$, then the minimum value of $\frac{4 \sin x \cos x+3}{\cos ^{2} x}$ is $\qquad$
$$
\begin{array}{l}
\frac{4 \sin x \cos x+3}{\cos ^{2} x}=\frac{3 \sin ^{2} x+4 \sin x \cos x+3 \cos ^{2} x}{\cos ^{2} x}=3 \tan ^{2} x+4 \tan x+3 \\
=3\left(\tan x+\frac{2}{3}\right)^{2}+\frac{5}{3} \geqslant \frac{5}{3}, \text { equality holds when } \tan x=-\frac{2}{3} .
\end{array}
$$