Try this drag a L across the 3 lines.
; multifillet 3 lines
; by Alanh Info@alanh.com.au Feb 2020
(defun c:mfillet2 (/ pt1 pt2 pt3 pt4 obj1 obj2 lst1 lst2 lst dist1 dist2 x ss ss2)
(setq pt1 (getpoint "\nPick outside"))
(setq pt2 (getpoint pt1 "\nPick inside"))
(setq pt3 (getpoint pt2 "\nPick outside"))
(setq rad (getreal "\nEnter inside radius"))
(setq lst (list pt1 pt2))
(setq ss (ssget "F" lst (list (cons 0 "*line"))))
(setq lst (list pt2 pt3))
(setq ss2 (ssget "F" lst (list (cons 0 "*line"))))
(if (= (sslength ss) (sslength ss2))
(progn
(command "line" pt1 pt2 "")
(setq obj1 (vlax-ename->vla-object (entlast)))
(command "Line" Pt2 pt3 "")
(setq obj2 (vlax-ename->vla-object (entlast)))
(setq lst1 '())
(repeat (setq x (sslength ss))
(setq obj (vlax-ename->vla-object (ssname ss (setq x (- x 1)))))
(setq pt4 (vlax-curve-getclosestpointto obj pt2))
(setq dist1 (distance pt2 pt4))
(setq lst1 (cons (list dist1 pt4) lst1))
)
(setq lst1 (vl-sort lst1
(function (lambda (e1 e2)
(< (car e1) (car e2))
)
)
)
)
(setq lst2 '())
(repeat (setq x (sslength ss2))
(setq obj (vlax-ename->vla-object (ssname ss2 (setq x (- x 1)))))
(setq pt4 (vlax-curve-getclosestpointto obj pt2))
(setq dist1 (distance pt2 pt4))
(setq lst2 (cons (list dist1 pt4) lst2))
)
(setq lst2 (vl-sort lst2
(function (lambda (e1 e2)
(< (car e1) (car e2))
)
)
)
)
(vla-delete obj1)
(vla-delete obj2)
(setq d1 (- (car (nth 1 lst2))(car(nth 0 lst2))))
(setq d2 (- (car (nth 2 lst2))(car(nth 1 lst2))))
(setq x 0)
(setvar 'filletrad rad)
(setq pt1 (cadr (nth x lst1)))
(setq pt2 (cadr (nth x lst2)))
(command "fillet" pt1 pt2)
(setvar 'filletrad (+ rad d1))
(setq x (+ x 1))
(setq pt1 (cadr (nth x lst1)))
(setq pt2 (cadr (nth x lst2)))
(command "fillet" pt1 pt2)
(setvar 'filletrad (+ rad d1 d2))
(setq x (+ x 1))
(setq pt1 (cadr (nth x lst1)))
(setq pt2 (cadr (nth x lst2)))
(command "fillet" pt1 pt2)
)
)
(princ)
)
(c:mfillet2)