I'm trying to calculate the circumference of a circle given say three reference points from a 2d coordinates that would form an arc.
The problem is the reference points may be slightly inaccurate so i'm using three to hopefully get an exact arc.
Once I have my 3 reference points how can I calculate the circumference?
I'm attaching an image so you may better understand what I'm trying to do.
 Also, the reason I'm doing it this way is that the image is a scan and top or side of the circle may be chopped off so getting the diameter may not always be possible and the size of the circle differs from time to time.