code: public void set(double x1, double y1, double z1, double x2, double y2, double z2, double x3, double y3, double z3) {
          this.MODE = TRIANGLE_3D;
          this.x1 = x1;
          this.y1 = y1;
          this.z1 = z1;
          this.x2 = x2;
          this.y2 = y2;
          this.z2 = z2;
          this.x3 = x3;
          this.y3 = y3;
          this.z3 = z3;
          calcG();
      }
docs: Sets x,y-coordinate of corners.
  
  @param x1 The x-coordinate of the first corner.
  @param y1 The y-coordinate of the first corner.
  @param z1 The z-coordinate of the first corner.
  @param x2 The x-coordinate of the second corner.
  @param y2 The y-coordinate of the second corner.
  @param z2 The z-coordinate of the second corner.
  @param x3 The x-coordinate of the third corner.
  @param y3 The y-coordinate of the third corner.
  @param z3 The z-coordinate of the third corner.