arc or curve fitting program

I'm taking some careful measurements on a part I'm reverse
engineering.

Given three, or more, points on a curve. How do I calculate the best
G02/G03 parameters to cut that curve? I'm hoping for a simple windows
app. that lets you put in points and gives the center and radius of
the circle fitting those points the best. Also, I'll need to be able
to calculate x,y points on that curve (you need endpoints in G02 and
I'll not be able to measure right at the endpoint). Is the formula
r**2 = (y - a)**2 + (x - b)**2 ?

I remember covering this in calculus class 30 years ago, but the only
reason I got through calculus was due to a very smart girl that let
me sit right next to her, very near her paper, during quizes.
Fortunately my eyes had great distance vision at that time <G>


Karl