3. In Rt $\triangle A B C$, $C B=3, C A=4, M$ is a moving point on the hypotenuse $A B$, and through $M$ draw $M D \perp A C$ at point $D$, and through $M$ draw $M E \perp C B$ at point $E$. Then the minimum value of line segment $D E$ is
3. $\frac{12}{5}$.

As shown in Figure 4, connect $C M$. From the given conditions, quadrilateral $C D M E$ is a rectangle, so $D E=C M$. Clearly, when $C M \perp A B$, $C M$ is the shortest. At this time, $C M=\frac{12}{5}$.