How to show 3 <li> in one row

I need to show 3 images in each row in <ul> tag. I have multiple <ul> tags and in each <ul> tag, I am showing 6 images. Now i have to show 3 <li> in first row and next 3 <li> in second row. Then same for next <ul>. I need to break row after 3 <li>. This is my code :

 <div class="sets">

    <?php  foreach ($sets as $set => $items) : ?>
        <ul class="set test-set">

        <?php  $i=0; foreach ($items as $thumb) : ?>

        <?php

            /* Prepare Image */
            $content = '<img src="'.$thumb['cache_url'].'" width="'.$thumb['width'].'" height="'.$thumb['height'].'" alt="'.$thumb['filename'].'" />';

        ?>
        <?php  if($i === 0): 
        echo '<li><div>'; ?>
        <?php endif; ?> 

        <?php echo $content; ?>

        <?php  if($i === 2):  $i = 0; ?>

        <?php else: $i++; endif; ?>
        </div></li>                 
        <?php endforeach; ?> 

    </ul>
    <?php endforeach; ?>

</div>