3. Given that $P$ is any point on the arc $\overparen{A D}$ of the circumcircle of square $A B C D$, find the value of $\frac{P A+P C}{P B}$.

 untranslated text:
已知 $P$ 是正方形 $A B C D$ 的外接圆 $\overparen{A D}$ 上任一点, 求 $\frac{P A+P C}{P B}$ 的值.