3つおきのdivのphpwhileループ変数

for ($i = 0; $i < $numRecords; $i++)
{
 $className = "";
 if (($i % 3) == 0)
 {
  $className = "last"
 }

 ....
}

ここで重要なのは($i % 3) == 0です。 。

編集： 以下はあなたのコメントへの返答です。

/* LOOP THROUGH SHOEDATA TABLE */

$results = mysql_query("SELECT * FROM shoeData");

$i = 0;
while($row = mysql_fetch_array($results)){
$i++;
$name = $row['name'];
$about = $row['about'];
$company = $row['company'];
$buy = $row['buy'];
$tags = $row['tags'];
$id = $row['id'];
$image = $row['image'];


/* ECHO THE SHOEDATA RESULTS */         
    $additionalClass = ($i % 3) == 0 ? " last" : "";
    echo "<div class='imageBorder span-8 column" . $additionalClass . "'>";
        echo "<div id='imageHeight'>";
        echo "<img  src='thumbs/$image'>";
        echo "</div>";

        echo "<ul>";

                echo "<li>$name</l1>";
                echo "<li>$about</l1>";
                echo "<li>$company</l1>";
                echo "<li><a href='$buy'>BUY</a></l1>";
                echo "<li>$tags</l1>";
        echo "</ul>";
    echo "</div>";


}/*SHOEDATA WHILE LOOP ENDS */