3. In $\triangle A B C$, it is known that $A B=c, A C=b, B C$ $=a$, the heights on sides $B C$ and $A C$ are $h_{a}$ and $h_{b}$, respectively, and $a \leqslant$ $h_{a}, b \leqslant h_{b}$. Find the degree measures of the three interior angles of $\triangle A B C$.
(1999, Tianjin City Junior High School Mathematics Competition)
(In any triangle, there are $h_{a} \leqslant b$, $h_{b} \leqslant a$. It is also easy to know that $a \leqslant h_{a} \leqslant b \leqslant h_{b} \leqslant a$, which means $a=b=h_{a}$ $=h_{b}$. Therefore, $\triangle A B C$ is an isosceles right triangle. Hence $\angle A=$ $\left.\angle B=45^{\circ}, \angle C=90^{\circ}.\right)$