Done very well, but you trapped into one definition lack:
The three ordinates may not be zero, because these special cases built up a cutplane parallel to one of the three axis.
Try this (copy it to
MASTERscript to get PRINT to work).
!-----SET SIZES-----
len=1.0
dep=1.0
wall_thk=0.090
!-----SET POINTS-----
x1=0.2 : y1=0.0 : z1=0.3
x2=1.0 : y2=0.5 : z2=0.6
x3=0.0 : y3=1.0 : z3=0.3
!-----CALCULATE VALUES FOR PLANAR EQUATION-----
co_a=(y1*(z2-z3))+(y2*(z3-z1))+(y3*(z1-z2))
co_b=(z1*(x2-x3))+(z2*(x3-x1))+(z3*(x1-x2))
co_c=(x1*(y2-y3))+(x2*(y3-y1))+(x3*(y1-y2))
co_d=-((x1*((y2*z3)-(y3*z2)))+(x2*((y3*z1)-(y1*z3)))+(x3*((y1*z2)-(y2*z1))))
!-----CALCULATE POINTS FOR CUTPLANE-----
IF co_a#0.0 THEN
x_cut=-co_d/co_a
ELSE
x_cut=0
ENDIF
IF co_b#0.0 THEN
y_cut=-co_d/co_b
ELSE
y_cut=0
ENDIF
IF co_c#0.0 THEN
z_cut=-co_d/co_c
ELSE
z_cut=0
ENDIF
!-----DRAW OBJECT WITH CUTPLANE-----
if x_cut and y_cut and z_cut THEN CUTPLANE x_cut, y_cut, z_cut, 0
PRISM_ 9, 5.0,
0.0, 0.0, 15,
len, 0.0, 15,
len, dep, 15,
0.0, dep, 15,
0.0, dep-wall_thk, 15,
len-wall_thk, dep-wall_thk, 15,
len-wall_thk, wall_thk, 15,
0.0, wall_thk, 15,
0.0, 0.0, -1
if x_cut and y_cut and z_cut THEN CUTEND
!-----DRAW SPANING PLANE-----
MATERIAL "Glas"
PLANE 3,
x1,y1,z1,
x2,y2,z2,
x3,y3,z3
!-----RECALCULATE POINTS IN PLANE-----
PRINT "If all Points are on the Plane, represented by the equation, all following results sould be zero."
PRINT "POINT 1: ",co_a*x1+co_b*y1+co_c*z1+co_d
PRINT "POINT 2: ",co_a*x2+co_b*y2+co_c*z2+co_d
PRINT "POINT 3: ",co_a*x3+co_b*y3+co_c*z3+co_d
PRINT "The constants a/b/c/d: ",co_a,"/",co_b,"/",co_c,"/",co_d
END
