i want to know how i can draw ellipse with the three points marked in the qgraphicsscene
It better be a circle (there will be infinitely many ellipses that pass through those points).

Read more about Circumscribed circle, all the required math is mentioned there. All you need to do is calculate the circle centre and radius, and use Qt API void drawEllipse(const QPoint & center, int rx, int ry);