Hey Kawaisin,
This should do the trick (untested):
<?php
if(have_rows("tombstones"))
{
$tombstonescount = 0;
while( have_rows("tombstones") )
{
$tombstonescount++;
$class = '';
$image = get_sub_field('tombstone_image');
if(i % 3 == 0)
{
$class .= 'last';
}
?>
<div class="one_third <?php echo $class;?>">
<img src="<?php echo $image['url']; ?>" alt="<?php echo $image['alt'] ?>" title="<?php echo $image['title'] ?>" />
</div>
<?php
}
}
?>