3. In trapezoid $A B C D$, $A B / / D C, \angle A=$ $90^{\circ}, A B=4, C D=3, B C=7, O$ is the midpoint of side $A D$, then the distance from $O$ to $B C$ is $\qquad$ .
3. $2 \sqrt{3}$
3. Hint: Let $M$ be the midpoint of $B C$: then $O M=\frac{7}{2}$. Connect $O B, O C$.