3. Given the vector $\boldsymbol{m}=(\sin B, 1-\cos B)$, and the angle between vector $\boldsymbol{m}$ and vector $\boldsymbol{n}=(2,0)$ is $\frac{\pi}{3}$, where $A, B, C$ are the interior angles of $\triangle A B C$, then the size of angle $B$ is $\qquad$.
3. $\frac{2}{3} \pi$ Detailed explanation: $\because m=(\sin B, 1-\cos B)$, and the angle formed with vector $\boldsymbol{n}=(2,0)$ is $\frac{\pi}{3}, \therefore \frac{1-\cos B}{\sin B}=\sqrt{3}$.
$\therefore \tan \frac{B}{2}=\sqrt{3}$, and since $0<B<\pi, \therefore \frac{B}{2}=\frac{\pi}{3}$, which means $B=\frac{2}{3} \pi, A+C=\frac{\pi}{3}$.