Offset from an arc

I am trying to write a routine that draws a line offset 3" from an arc and I
am not sure where to start. I know that for a line you can do:

(setq a (angle pt1 pt2 pt3)
pt11 (polar pt1 (+ a (dtr 90)) -3)
pt12 (polar pt2 (+ a (dtr 90)) -3)
pt13 (polar pt3 (+ a (dtr 90)) -3)
)

How would I accomplish the same thing on the curve after having the user
pick the three points on the arc?

 

Never mind ... found a work around (unless there is a better way)

(setq pt1 (getpoint "\nPick first point..."))
(if pt1 (setq pt2 (getpoint "\nPick point on curve...")))
(if pt2 (setq pt3 (getpoint "\nPick end point...")))
(command "arc" pt1 pt2 pt3 ))
(setq $arc (ssget "l"))
(setq $arcp (getpoint "\nPick side of curve to hatch..."))
(command "offset" "3" $arc $arcp "")
(command "erase" $arc "")

 

Here are two way it can be done.
(defun c:arcoffset (/ useros pt1 pt2 pt3 vobj)
(setq useros (getvar "osmode"))
(setvar "osmode" 0)
(if (and (setq pt1 (getpoint "\nPick first point..."))
(setq pt2 (getpoint "\nPick point on curve..."))
(setq pt3 (getpoint "\nPick end point..."))
)
(progn
(command "arc" pt1 pt2 pt3)
(setq vobj (vlax-ename->vla-object (entlast)))
(vlax-invoke vobj 'offset 3)
(vlax-invoke vobj 'delete)
)
)
(setvar "osmode" 0)
(princ)
)

(defun c:arcoffset2 (/ useros vobj entold entnew)
(setq useros (getvar "osmode")
entold (entlast))
(setvar "osmode" 0)
(command "arc" pause pause pause)
(if (not (eq entold (setq entnew (entlast))))
(progn
(setq vobj (vlax-ename->vla-object entnew))
(vlax-invoke vobj 'offset 3)
(vlax-invoke vobj 'delete)
)
)
(setvar "osmode" 0)
(princ)
)