3. Given points $A(1,2)$ and $B(3,4)$, there is a point $P$ on the coordinate axis, and $P A + P B$ is minimized. Then the coordinates of point $P$ are $\qquad$.
3. $\left(0, \frac{5}{2}\right)$.

Using knowledge of axis symmetry, ruler and compass constructions, and linear functions, we know that the minimum value of $P A + P l 3$ occurs when $P$ is on the $y$-axis, which is smaller than when $P$ is on the $x$-axis. Thus, the coordinates of $P$ are $\left(0, \frac{5}{2}\right)$.