code: function ( vectorA, vectorB, vectorC ) {
  
  		var a, b, c,
  			point = new THREE.Vector3(),
  			tmp = THREE.GeometryUtils.__v1;
  
  		a = THREE.GeometryUtils.random();
  		b = THREE.GeometryUtils.random();
  
  		if ( ( a + b ) > 1 ) {
  
  			a = 1 - a;
  			b = 1 - b;
  
  		}
  
  		c = 1 - a - b;
  
  		point.copy( vectorA );
  		point.multiplyScalar( a );
  
  		tmp.copy( vectorB );
  		tmp.multiplyScalar( b );
  
  		point.addSelf( tmp );
  
  		tmp.copy( vectorC );
  		tmp.multiplyScalar( c );
  
  		point.addSelf( tmp );
  
  		return point;
  
  	}
docs: Get random point in triangle (via barycentric coordinates) (uniform distribution) http://www.cgafaq.info/wiki/Random_Point_In_Triangle