<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<script language="javascript">
function Verifica(){
if(form.txt1.value == ''){
alert('Llene el campo uno')
form.txt1.focus()
exit()
}
if(form.txt2.value == ''){
alert('Llene el campo dos')
form.txt2.focus()
exit()
}
if(form.txt3.value == ''){
alert('Llene el campo tres')
form.txt3.focus()
exit()
}
form.submit()
}
</script>
<title>Documento sin título</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
</head>
<body>
<form name="form" method="post" action="prueba2.htm">
<p>
<input name="txt1" type="text" id="txt1">
</p>
<p>
<input name="txt2" type="text" id="txt2">
</p>
<p>
<input name="txt3" type="text" id="txt3">
</p>
<p>
<input type="button" name="Submit" value="Enviar" onclick="Verifica()">
</p>
</form>
<p>Bueno ojala te sirva, bye</p>
</body>
</html>
Este archivo se abre si la Validacion esta ok
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>Documento sin título</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
</head>
<body>
Se lleno los tres campos
</body>
</html>