I want to open a new window by clicking on a marker using Google Maps API 3.
Unfortunately there are not many examples for the Google Maps API and I found out this c
url isn't an object on the Marker class.  But there's nothing stopping you adding that as a property to that class.  I'm guessing whatever example you were looking at did that too.  Do you want a different URL for each marker?  What happens when you do:
for (var i = 0; i < locations.length; i++) 
{
    var flag = new google.maps.MarkerImage('markers/' + (i + 1) + '.png',
      new google.maps.Size(17, 19),
      new google.maps.Point(0,0),
      new google.maps.Point(0, 19));
    var place = locations[i];
    var myLatLng = new google.maps.LatLng(place[1], place[2]);
    var marker = new google.maps.Marker({
        position: myLatLng,
        map: map,
        icon: flag,
        shape: shape,
        title: place[0],
        zIndex: place[3],
        url: "/your/url/"
    });
    google.maps.event.addListener(marker, 'click', function() {
        window.location.href = this.url;
    });
}