wordpress form filtering help

Im trying to use a form to filter my posts by some custom fields.
The issue at the moment is that its only using the last filter query and ignoring the first two.

I havent done php for a year or two so I’ve forgotten everything.
Any tips as to what I’m doing wrong, and if there is a better way to go about it, please let me know.

If there is a plugin that would be suited to solved this issue please let me know. I intended to use, WP-Smart-Sort-Premium, but its creator has gone awol and its no longer for sale…

Thank you.

<?php
/*
Template Name: Events v2.0
*/
?>

<?php
//Filter links

?>
<form action="" method="post">
<input name="platform" value="Xbox 360" type="checkbox"
id="commfees">Xbox 360 <br />
<input name="platform" value="PC" type="checkbox"
id="commfees">PC <br />
<input name="game" value="Halo 3" type="checkbox"
id="commfees">Halo 3 <br />
<input name="game" value="Modern Warfare 2" type="checkbox"
id="commfees">Modern Warfare 2 <br />
<input name="prize" value=">=0" type="checkbox"
id="commfees">$0 Prize <br />
<input name="prize" value=">=1" type="checkbox"
id="commfees">$1+ Prize <br />
<input name="prize" value=">=1000" type="checkbox"
id="commfees">$1000+ Prize <br />

<input type="submit" value="Submit" />

</form>

<?php
//setting filter values
//platform
if(isset($_POST['platform'])) {
$var = $_POST['platform'];
$platform = 'meta_key=Platform&meta_value='.$var.'';
} else {
$platform = 'meta_key=Platform';
}

//Game
if(isset($_POST['game'])) {
$var = $_POST['game'];
$game = 'meta_key=Game&meta_value='.$var.'';
} else {
$game = 'meta_key=Game';
}

//Prize
if(isset($_POST['prize'])) {
$var = $_POST['prize'];
$prize = 'meta_key=TotalPrizeValue&meta_value'.$var.'';
} else {
$prize = 'meta_key=TotalPrizeValue';
}

?>

<?php
//query string
$query = $platform .'&'. $game .'&'. $prize;
query_posts($query);

 ?>