(3) If in $\triangle A B C$, $A$ and $B$ are acute angles, and $\sin ^{2} A+\sin ^{2} B=\sin C$, then the most accurate description of the shape of $\triangle A B C$ is ( ).
(A) Right triangle
(B) Isosceles triangle
(C) Isosceles right triangle
(D) None of the above
(3) A Hint: If $A+B>\frac{\pi}{2}$, then $\sin A>\cos B, \sin B>\cos A$, thus
$$
\sin ^{2} A+\sin ^{2} B>\sin A \cos B+\sin B \cos A=\sin (A+B)=\sin C,
$$

which contradicts the given condition. Similarly, if $A+B<\frac{\pi}{2}$, it also leads to a contradiction. Therefore, $A+B=\frac{\pi}{2}$.