Unity - Scripting API:

public function Set3Points(a: Vector3, b: Vector3, c: Vector3): void;

public void Set3Points(Vector3 a, Vector3 b, Vector3 c);

Parameters

a	First point in clockwise order.
b	Second point in clockwise order.
c	Third point in clockwise order.

Sets a plane using three points that lie within it. The points go around clockwise as you look down on the top surface of the plane.