nth-child stops working after some time - html

Question

This is how my code looks like. I removed some parts. So, don't worry if you see some syntax error or something like that.
<?php for($i = 0; $i < $num; $i++){ ?>
<div id="my-ads-block">
<div id="my-ads-name">
<?php echo $title[$i]; ?>
</div>
<div id="my-ads-content">
<div id="my-ads-picture">
<a href="<?php echo $picture[$i] ?>">
<img src="/<?php echo $picture[$i] ?>">
</a>
</div>
<div id="my-ads-menu">
<div class="my-ads-buttons"><span>Posted Date: </span><?php echo date_format($date[$i],"F j, Y") ?></div>
<div class="my-ads-buttons"><span>Price: </span><?php setlocale(LC_MONETARY, 'en_CA'); echo money_format('$%!i', $price[$i]); ?></div>
<div class="my-ads-buttons">Edit This Ad</div>
<div class="my-ads-buttons">Delete This Ad</div>
<div class="my-ads-buttons">Open This Ad</div>
</div>
</div>
</div>
<?php } ?>
This is the css
#my-ads-block{
width: 250px;
margin-right: 7px;
float: left;
}
#my-ads-block:nth-child(3n){
margin-right: 0;
}
I print small boxes which contains some informations and one page can have 3 boxes next to each other. So, I would like to explain with some examples.
The code above, should print columns like this
1 2 3
4 5 6
7 8 9
But the problem is, it prints like this:
1 2 3
4 5 6
7
8 9 10
So, after 6th element, something goes wrong and I cannot figure out why. Thank you for your help.