3. As shown in Figure 1, in $\triangle A B C$, $D$ and $E$ are points on $A C$ and $A B$ respectively. Connect $D E$, $B D$, and $C E$, and let $O$ be the intersection of $B D$ and $C E$. If we set $m$
$$
=\frac{S_{\triangle A D E}}{S_{\triangle A B C}}, n=\frac{S_{\triangle O D E}}{S_{\triangle O B C}} \text {, }
$$

then the relationship between $m$ and $n$ is
( ).
(A) $m>n$
(B) $m=n$
(C) $m<n$
(D) cannot be determined, depends on the positions of points $D$ and $E$
3. B.

From the formula for the area of a triangle, we have
$$
m=\frac{S_{\triangle A D E}}{S_{\triangle A B C}}=\frac{\frac{1}{2} A D \cdot A E \sin A}{\frac{1}{2} A B \cdot A C \sin A}=\frac{A D \cdot A E}{A B \cdot A C} .
$$

Similarly, $n=\frac{S_{\triangle O D E}}{S_{\triangle O B C}}=\frac{O D \cdot O E}{O B \cdot O C}$.
Connecting $A O$, we have
$$
\begin{array}{l}
\frac{O D}{O B} \cdot \frac{O E}{O C}=\frac{S_{\triangle A O D}}{S_{\triangle A O B}} \cdot \frac{S_{\triangle A O E}}{S_{\triangle A O C}} \\
=\frac{S_{\triangle A O E}}{S_{\triangle A O B}} \cdot \frac{S_{\triangle A O D}}{S_{\triangle A O C}}=\frac{A E}{A B} \cdot \frac{A D}{A C} .
\end{array}
$$

Therefore, $m=n$.