3. In Rt $\triangle A B C$, $D$ is the midpoint of the hypotenuse $B C$, and $E, F$ are points on $A B, A C$ respectively. Let the perimeter of $\triangle D E F$ be $l$. Then $(\quad)$.
(A) $l>B C$
(B) $l=B C$
(C) $l<B C$
(D) cannot be determined
3. A.

As shown in Figure 4, construct the symmetric points $G$ and $H$ of $D$ with respect to $AB$ and $AC$, respectively, and connect $AH$, $AG$, $AD$, $HF$, and $GE$.
Then, $DE = GE$, $DF = HF$, and $AD = AG = AH$.
It is easy to prove that points $H$, $A$, and $G$ are collinear, and $\triangle HDG$ is a right triangle.
In quadrilateral $HFEG$, we have
$$
HF + FE + EG > HC.
$$

Therefore, $FD + FE + DE > BC$, which means $l > BC$.