Dynamically Update Dropdowns Using PHP

I have been using PHP for a little while but just getting into JavaScript and really need to dynamically update dropdown boxes read from a mySQL database. There are three dropdown boxes, the first reads in fine. The second dropdown box then gets populated using the selection from the first as the condition in mySQL and the same again for the third.The page refreshes itself after the selection from the first box using JavaScript and populates the second using the PHP _GET method (this works fine). The issue is when I need to do this again for the second time because my JS is only set up to do it once and I don't no how to amend it. Also it will have to do it a third time because I need that info to complete the form and update the DB.

<SCRIPT language=JavaScript>
function reload(form){
var val=form.cat.options[form.cat.options.selectedIndex].value;
self.location='bookf.php?cat='+val ;
}
</script>

echo "<form method=post name=f1 action=''>";
////////// Starting of first drop downlist /////////
echo "<select name='cat' onchange="reload(this.form)"><option value=''>Select a Film</option>";
while($noticia2 = mysql_fetch_array($quer2)) {
if($noticia2['FilmID']==@$cat){echo "<option selected value='$noticia2[FilmID]'>$noticia2[FilmTitle]</option>"."<BR>";}
else{echo "<option value='$noticia2[FilmID]'>$noticia2[FilmTitle]</option>";}
}

echo "</select><br />";
////////////////// This will end the first drop down list ///////////
////////// Starting of second drop downlist /////////
echo "<select name='subcat' onchange="reload(this.form)"><option value=''>Select a Date</option>";
while($noticia = mysql_fetch_array($quer)) {
if($noticia2['FilmID']==@$cat){echo "<option selected value='$noticia2[FilmID]'>$noticia2[Date]</option>"."<BR>";}
else{echo "<option value='$noticia[FilmID]'>$noticia[Date]</option>";}
}
echo "</select><br />";
////////////////// This will end the second drop down list ///////////

////////// Starting of third drop downlist /////////
echo "<select name='subcat2' onchange="reload(this.form)"><option value=''>Select a Time</option>";
while($noticia = mysql_fetch_array($quer3)) {
if($noticia2['FilmID']==@$cat){echo "<option selected value='$noticia2[FilmID]'>$noticia2[Time]</option>"."<BR>";}
else{echo "<option value='$noticia[FilmID]'>$noticia[Time]</option>";}
}
echo "</select><br />";
////////////////// This will end the third drop down list ///////////