If you set width to 100% as well you'll get them stacked one above the other but you'll have to scroll:
<html>
<head>
<title>Zorro Panel</title>
<style type="text/css">
iframe {
width: 100%;
height: 100%;
}
</style>
</head>
<body>
<iframe src="Z12.htm"></iframe>
<iframe src="Z3.htm"></iframe>
<iframe src="Z7.htm"></iframe>
</body>
</html>
If you want them all visible but half height, you can set width to 100% and height to 33%:
<html>
<head>
<title>Zorro Panel</title>
<style type="text/css">
iframe {
width: 100%;
height: 33%;
}
</style>
</head>
<body>
<iframe src="Z12.htm"></iframe>
<iframe src="Z3.htm"></iframe>
<iframe src="Z7.htm"></iframe>
</body>
</html>