A proposal with GEOMCAL, pending VLisp
** DATA **
(setq a1 (list 3.69631915 2.65184043 4.69631915))
(setq a2 (list 5 2 6))
(setq b1 (list 1 1 0))
(setq b2 (list 2.36080723 2.36080723 2.72161446))
(setq c (inters a1 a2 b1 b2 nil))
Fillet radius = 1.0
(cal "a3=pld(c,a1,1.0)")
(cal "b3=pld(c,b1,1.0)")
(cal "pm=plt(a3,b3,0.5)")
(cal "d1=dpl(pm,c,a1)")
(setq d2 (distance pm c))
(setq d3 (sqrt (- (expt d2 2) (expt d1 2))))
(setq d (/ d3 d1))
(setq dc (sqrt (+ (expt d 2) 1.0)))
;**** ARC ****
;p1 = start
(cal "p1=pld(c,a1,d)")
;p2 = end
(cal "p2=pld(c,B1,d)")
;pc = center
(cal "pc=pld(c,pm,dc)")