3. As shown in the figure, $\triangle A B C$ is an acute triangle, $B E \perp A C, C F \perp A B$. Then the value of $S_{\triangle A E F}=S_{\triangle A B C}$ is ( ).
(A) $\sin A$
(B) $\cos A$
(C) $\sin ^{2} A$
(D) $\cos ^{2} A$
3. (D).
$$
\begin{array}{l}
\text { In Rt } \triangle A B E, \frac{A E}{A B}=\cos A, \\
\text { In Rt } \triangle A C F, \frac{A F}{A C}=\cos A, \\
\therefore \frac{S_{\triangle A E F}}{S_{\triangle A B C}}=\frac{\frac{1}{2} A E \cdot A F \sin A}{\frac{1}{2} A B \cdot A C \sin A}=\cos ^{2} A .
\end{array}
$$