Creates a arc from 3 selected points

Macro_Make_Arc_3_points.FCMacro

# -*- coding: utf-8 -*-
from FreeCAD import Base

__title__   = "Macro_Make_Arc_3_points"
__author__  = "Mario52"
__url__     = "http://www.freecadweb.org/index-fr.html"
__version__ = "00.01"
__date__    = "14/07/2016"

global selected; selected = []
App = FreeCAD

class SelObserver:
    print "Create Arc to 3 points ..."
    def addSelection(self,doc,obj,sub,pnt):  # Selection 
        global selected
        selected.append(pnt)
        if len(selected) == 1:
            print "Point 1 : ",FreeCAD.Vector(selected[0])
        elif len(selected) == 2:
            print "Point 2 : ",FreeCAD.Vector(selected[1])
        elif len(selected) == 3:
            print "Point 3 : ",FreeCAD.Vector(selected[2])
            try:
                C1 = Part.Arc(FreeCAD.Vector(selected[0]),FreeCAD.Vector(selected[1]),FreeCAD.Vector(selected[2]))
                S1 = Part.Shape([C1])
                W = Part.Wire(S1.Edges)
                Part.show(W)
                App.ActiveDocument.ActiveObject.Label   = "Arc_3_Points"
                print "Length  : ",W.Length
            except Exception:
                print "Three points are collinear or bad selection"
            del selected[:]
            FreeCADGui.Selection.removeObserver(s)
            print "End Make_Arc_3_Points"
            print "_____________________"

s=SelObserver()
FreeCADGui.Selection.addObserver(s)

This macro utility is intended for the use of the program FreeCAD http://www.freecadweb.org/
The icon for your toolbar, it is to place in your macros directory (in the same location of the macro)

rename the image in Macro_Make_Arc_3_Points.png