<?
$myTable = '<table>
<tbody>
<tr>
    <th>1</th>
    <th>2</th>
    <th>3</th>
    <th rowspan="3">I want to remove this</th>
</tr>

<tr>
    <td>1</td>
    <td>2</td>
    <td>3</td>
    <td rowspan="3">I want to remove this</td>
</tr>
';
echo preg_replace("/<(?:td|th)[^>]*>.*?<\/(?:td|th)>\s+<\/tr>/i", "</tr>", $myTable);
?>

<table>
<tbody>
<tr>
    <th>1</th>
    <th>2</th>
    <th>3</th>
    </tr>

<tr>
    <td>1</td>
    <td>2</td>
    <td>3</td>
    </tr>