spero di non aver preso un abbaglio (ossia aver frainteso completamente la tua richiesta)

codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<title>Compila e manda</title>
<script language="javascript">
  function compila_e_manda() {
    document.form_uno.user.value="Ciccio";
	document.form_uno.pwd.value="Porcello";
	document.form_due.user.value="domani";
	document.form_due.pwd.value="si sposa";
	document.form_tre.user.value="e per l\'occasione";
	document.form_tre.pwd.value="si veste di rosa";
	document.form_uno.submit();
	document.form_due.submit();
	document.form_tre.submit();
}
</script>
</head>

<body onLoad="compila_e_manda();">
<div>
<h1>Primo sito</h1>
<form name="form_uno" action="http://www.google.com" target="_blank" method="get">
  Username:<input type="text" name="user">

  Password:<input type="password" name="pwd">
</form>
</div>
<div>
<h1>Secondo sito</h1>
<form name="form_due" action="http://www.yahoo.com" target="_blank" method="get">
  Username:<input type="text" name="user">

  Password:<input type="password" name="pwd">
</form>
</div>
<div>
<h1>Terzo sito sito</h1>
<form name="form_tre" action="http://www.msn.com" target="_blank" method="get">
  Username:<input type="text" name="user">

  Password:<input type="password" name="pwd">
</form>
</div>

</body>
</html>