code: function ( vectorA, vectorB, vectorC ) {
  
  		var s, a, b, c,
  			tmp = THREE.GeometryUtils.__v1;
  
  		tmp.sub( vectorA, vectorB );
  		a = tmp.length();
  
  		tmp.sub( vectorA, vectorC );
  		b = tmp.length();
  
  		tmp.sub( vectorB, vectorC );
  		c = tmp.length();
  
  		s = 0.5 * ( a + b + c );
  
  		return Math.sqrt( s * ( s - a ) * ( s - b ) * ( s - c ) );
  
  	}
docs: Get triangle area (by Heron's formula) http://en.wikipedia.org/wiki/Heron%27s_formula