You need the normal, and a distance along the normal:

#macro plane3pts(a,b,c)
  #local norm=vnormalize(vcross(b-a,c-a));
  #local dst=vdot(a,norm);
  plane{norm, dst}
#end

You may want to change the first line of the macro to (negate norm):
  #local norm=-vnormalize(vcross(b-a,c-a));

One way will use right-hand-rule, the other left-hand-rule (counter-clockwise
vs clockwise order of points)... I'm not sure which is which, though. ;-)

-Nathan Kopp

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