aqui dejo el script que estoy probando:
<html>
<head>
<title>prueba</title>
<script language="Javascript" type="text/javascript">;
<!--
function escrito(num1, num2, num3){
document.getElementById('mensaje').innerHTML="Has elegido el número "+num3+".";
}
document.write(num1, num2, num3);
//-->
</script>
</head>
<body>
<form name="formulario" method="post" action="">
<select name="select1" size="1" id="select1">
<option value="0k">0k</option>
<option value="20k">20k</option>
<option value="30k">30k</option>
</select>
<select name="select2" size="1" id="select2">
<option value="1v">1v</option>
<option value="2v">2v</option>
</select>
<select name="select3" size="1" id="select3">
<option value="10k">10k</option>
<option value="50k">50k</option>
</select>
</form>
<input type="button" value="pinchame" onclick="escrito(document.getElementById('select1' ).value, document.getElementById('select2').value, document.getElementById('select3').value);">
</form>
<div id="mensaje"></div>
</body>
</html>