Среднее значение для трех TrackBar

<?php
if(isset($_POST['go'])) echo $_POST['trackbar'];
?>
<form method="POST" action="trak.php">
<input type="range" name="trackbar" />
<input type="submit" value="ok" name="go" />

<form>
    <input type="range" name="tb1"><br>
    <input type="range" name="tb2"><br>
    <input type="range" name="tb3"><br>
    <input type="submit"><br>
</form>
 
<?php
if(isset($_GET['tb1'], $_GET['tb2'], $_GET['tb3']))
{
    $average = ($_GET['tb1'] + $_GET['tb2'] + $_GET['tb3'])/3;
    echo "Среднее = ".$average;
}

<script type="text/javascript">
function showValue(id, newValue)
{
    document.getElementById("range"+id).innerHTML=newValue;
}
</script>
 
<form>
    <input type="range" name="tb1" value="0" onchange="showValue(1, this.value)"><br>
    <span id="range1">0</span><br>
    <input type="range" name="tb2" value="0" onchange="showValue(2, this.value)"><br>
    <span id="range2">0</span><br>
    <input type="range" name="tb3" value="0" onchange="showValue(3, this.value)"><br>
    <span id="range3">0</span><br>
    <input type="submit"><br>
</form>
 
<?php
if(isset($_GET['tb1'], $_GET['tb2'], $_GET['tb3']))
{
    $average = ($_GET['tb1'] + $_GET['tb2'] + $_GET['tb3'])/3;
    echo "Среднее = ".$average;
}