From the plot I think I found that:

z=1-x-y

S1) Has points (1,0,0), (0,1,0), (0,0,1) Plane Equation Becomes -x-y-z=0

S2) Has points (0,0,0), (1,0,0), (0,0,1) Plane Equation Becomes y=0

S3) Has Points (0,0,0), (0,1,0), (0,0,1) Plane Equation Becomes -x=0

S4) Has Points (0,0,0), (1,0,0), (0,1,0) Plane Equation Becomes -z=0

any idea what the next step should be?