Event.observe(window, 'load', function (event) {
     
      var altoMaximo = 0;

      if ($('main_left').getHeight() > altoMaximo) altoMaximo=$('main_left').getHeight();
      if ($('main_center').getHeight() > altoMaximo) altoMaximo=$('main_center').getHeight();
      if ($('main_right').getHeight() > altoMaximo) altoMaximo=$('main_right').getHeight();

      $('main_left').setStyle({
         height: altoMaximo + 'px'
      });

      $('main_center').setStyle({
         height: altoMaximo + 'px'
      });

      $('main_right').setStyle({
         height: altoMaximo + 'px'
      });

});