Hola de nuevo.
Te he preparado un código, a ver si te sirve:
Código PHP:
<html>
<head>
<script type="text/javascript">
function calcular(frm) {
n1 = parseFloat(frm.nota1.value);
n2 = parseFloat(frm.nota2.value);
n3 = parseFloat(frm.nota3.value);
if(isNaN(n1)) n1=0;
if(isNaN(n2)) n2=0;
if(isNaN(n3)) n3=0;
frm.total.value = n1*.4 + n2*.3 + n3*.3;
}
</script>
</head>
<body>
<form>
<input type="text" name="nota1" onkeyup="calcular(this.form)" />
<input type="text" name="nota2" onkeyup="calcular(this.form)" />
<input type="text" name="nota3" onkeyup="calcular(this.form)" />
<input type="text" name="total" readonly="readonly" />
</form>
</body>
</html>
Saludos,