hmmm...

package  
{
    
    import flash.display.*;
    import flash.geom.*;
    import com.greensock.*;
    import com.greensock.easing.*;


    public class Circle3 extends Sprite
    {
        public var p1:Point = new Point (465 * Math.random (), 465 * Math.random ());
        public var p2:Point = new Point (465 * Math.random (), 465 * Math.random ());
        public var p3:Point = new Point (465 * Math.random (), 465 * Math.random ());
        public var v1:Point = new Point (Math.random () - Math.random (), Math.random () - Math.random ());
        public var v2:Point = new Point (Math.random () - Math.random (), Math.random () - Math.random ());
        public var v3:Point = new Point (Math.random () - Math.random (), Math.random () - Math.random ());

       
        public function Circle3 () 
        {
            
            v1.normalize (5);
            v2.normalize (5);
            v3.normalize (5);

            addEventListener ("enterFrame", loop);
       
        }

        public function loop (e:*):void
        {// Move points around randomly
            
            p1 = p1.add (v1);
            p2 = p2.add (v2);
            p3 = p3.add (v3);
            
            var mainTL:TimelineMax = new TimelineMax({timeScale:1});
            mainTL.append

            if ((p1.x < 0) || (p1.x > 465)) 
            { p1.x = Math.max (0, Math.min (465, p1.x)); v1.x *= -1; }
            
            if ((p1.y < 0) || (p1.y > 465)) 
            { p1.y = Math.max (0, Math.min (465, p1.y)); v1.y *= -1; }
            
            
            if ((p2.x < 0) || (p2.x > 465)) 
            { p2.x = Math.max (0, Math.min (465, p2.x)); v2.x *= -1; }
            
            if ((p2.y < 0) || (p2.y > 465)) 
            { p2.y = Math.max (0, Math.min (465, p2.y)); v2.y *= -1; }
            
            
            if ((p3.x < 0) || (p3.x > 465)) 
            { p3.x = Math.max (0, Math.min (465, p3.x)); v3.x *= -1; }
            
            if ((p3.y < 0) || (p3.y > 465)) 
            { p3.y = Math.max (0, Math.min (465, p3.y)); v3.y *= -1; }
            
            // prepare graphics
            graphics.clear ();
            
            // find circle center
            var o:Object = findCircleCenter (p1, p2, p3);
            var c:Point = o.center;
            
            if (o.finite) {
                // find circle radius
                var r:Point = c.subtract (p1);
                // draw circle
                
                var sky: uint = 0x0077ff;
                var land: uint = 0x007700;
                
                graphics.beginFill (o.inside ? sky: land);
                graphics.drawRect (0, 0, 475, 475);
                graphics.endFill ();
                graphics.beginFill (o.inside ? land: sky);
                graphics.drawCircle (c.x, c.y, r.length);
                graphics.endFill ();
            } else {
                // draw line
                graphics.lineStyle (0, o.inside ? 0xff0000 : 0x0000ff);
                var line:Point = p2.subtract (p1);
                if (line.length == 0) {
                    line = p2.subtract (p3);
                    if (line.length == 0) {
                        line.x = 1;
                    }
                }

                line.normalize (1000);

                graphics.moveTo (p2.x - line.x, p2.y - line.y);
                graphics.lineTo (p2.x + line.x, p2.y + line.y);
            }

            // draw points
            graphics.lineStyle ();
            graphics.beginFill (0xFF0000);
            graphics.drawCircle (p1.x, p1.y, 3.5);
            graphics.drawCircle (p2.x, p2.y, 3.5);
            graphics.drawCircle (p3.x, p3.y, 3.5);
        }

        /*
        * Finds circle center from three points.
        * @see http://mathforum.org/library/drmath/view/54323.html
        */
        public function findCircleCenter (p1:Point, p2:Point, p3:Point): Object
        {
            var bc:Number = (p1.length * p1.length - p2.length * p2.length) * 0.5;
            var cd:Number = (p2.length * p2.length - p3.length * p3.length) * 0.5;
            var det:Number = (p1.x - p2.x) * (p2.y - p3.y) - (p2.x - p3.x) * (p1.y - p2.y);

            if (det == 0) {
                // center at infinity
                return { finite: false, inside: false, center: null };
            }
            
            return { finite: true, inside: det < 0, center: new Point (
                (bc * (p2.y - p3.y) - cd * (p1.y - p2.y)) / det,
                (cd * (p1.x - p2.x) - bc * (p2.x - p3.x)) / det
            )};
        }

    }

}