When you're given 3 points, (the origin being the 3rd), use the distance formula of (x1-x2)²+(y1-y2)² and use simultaneous equation on the three points. That way, you can get x and y. Afterwards, find the radius. Remember that the equation is (x-centre x)²+(y-centre y)² = R², not R. To find R, just use the distance formula but with the centre of the point and any point touching the circle. Hope this helps.