;| 3DARC.lsp 
draws a 3-point arc in 3D regardless of current UCS or view

by Bill Gilliss
bill at realerthanreal dot com
Comments and suggestions always welcome.

No warranty, either expressed or implied, is made as to the fitness of this
information for any particular purpose.  All materials are to be considered
'as-is', and use thereof should be considered as at your own risk.

ver 1.0 Jan 25 2010 - initial public release

Keywords: AutoCAD AutoLISP arc 3D
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(defun c:3darc ( / p1 p2 p3 p1w p2w p3w) 
(setq p1 (getpoint "Start point of arc:")
			p2 (getpoint p1 "\nMiddle point:")
			p3 (getpoint p2 "\nEnd point: ")
	    )
(if 
  (not (equal (distance p1 p3) (+ (distance p1 p2) (distance p2 p3))))
  (progn
    (setq p1w (trans p1 1 0)
          p2w (trans p2 1 0)
          p3w (trans p3 1 0)
          )
    (command "._ucs" "_3point" p1 p3 p2)
    (command "._arc" (trans p1w 0 1) (trans p2w 0 1) (trans p3w 0 1))
    (command "._ucs" "_p")  
    )
  (progn
    (if (vl-load-com) (vlr-beep-reaction))
    (prompt "\nError: points are co-linear.")
    )
  )
(princ)
)

(prompt "3DARC loaded.")
(princ)