Arc based on Start/End points and Radius- Mostly done

[Nialsh]

If I'm not mistaken, you're on the wrong track. Constructing an arc from two points and a radius is a very different problem from the three-point construction.

Given two points and a radius, there are 4 different arcs you can make (that lie on 2 circles). Here's some code that finds the centers of the 2 circles that the arcs can lie on:

//points (x1,y1) and (x2,y2)
//radius r
var mx = (x1+x2)/2;
var my = (y1+y2)/2;

var leg1x = mx-x1;
var leg1y = my-y1;
var leg1 = Math.sqrt(leg1x*leg1x + leg1y*leg1y);

if(leg1 > Math.abs(r))
	return; //no solution

var leg2 = Math.sqrt(r*r - leg1*leg1);
var leg2x = leg1y*leg2/leg1;
var leg2y = -leg1x*leg2/leg1;

var c1x = mx+leg2x;
var c1y = my+leg2y;

var c2x = mx-leg2x;
var c2y = my-leg2y;

You can probably work from that to construct the arc you want.