3. The three vertices of a triangle are $\left(a \sin \theta_{1}, a \cos \theta_{1}\right),\left(a \sin \theta_{2}, a \cos \theta_{2}\right)$, $\left(a \sin \theta_{3}, a \cos \theta_{3}\right)(a>0)$, then the coordinates of the circumcenter of the triangle are $\qquad$