something like this?
Code:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Untitled Document</title>
</head>
<body>
<form>
<select name="category" id="category">
<option value="1">cat name 1</option>
<option value="2">cat name 2</option>
<option value="3">cat name 3</option>
</select>
<select name="number_on_page" id="number_on_page">
<option value="1">cat name 1</option>
<option value="2">cat name 2</option>
<option value="3">cat name 3</option>
</select>
<select name="order" id="order">
<option value="1">order 1</option>
<option value="2">order 2</option>
<option value="3">order 3</option>
</select>
<input type="button" name="go" value="GO" onclick="navigate()" />
</form>
<script type="text/javascript">
function navigate()
{
var
s1 = document.getElementById('category')
,s2 = document.getElementById('number_on_page')
,s3 = document.getElementById('order')
;
var
sel1 = s1.options[s1.selectedIndex].value
,sel2 = s2.options[s2.selectedIndex].value
,sel3 = s3.options[s3.selectedIndex].value
;
// validation?
// to do...
//else
var base_url = "http://website.com";
base_url += "?cat="+sel1+"&nop="+sel2+"&order"+sel3;
// redirect
//window.location.href = base_url;
// DEBUG MODE:
alert(base_url);
}
</script>
</body>
</html>