Creating a 3D mashup with Virtual Earth is very easy.  Here is some sample code for a basic Virtual Earth 3D Mashup:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
<head>
<title>Virtual Earth 3D Mashup</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<script src="http://dev.virtualearth.net/mapcontrol/mapcontrol.ashx?v=6.2"></script>

<script>
function CreateMap()
{
   var map = new VEMap('myMap');
   map.LoadMap(new VELatLong(46.935893, -99.71), 4, VEMapStyle.Aerial, false, VEMapMode.3D);
}
</script>
</head>
<body onload="CreateMap();" style="width:100%;height:100%;">
<div id='myMap' style="position:absolute;width:100%;height:100%;"></div>
</body>
</html>

Click Here to see a live demo of this mashup.