How to insert an ad after every 3rd entry.

I created a form with a repeater field that has a photo upload and a few other small text fields. When a user submits a photo it is immediately added to a bootstrap slider. What I would like to do is add advertisements after every 3rd slide. I thought [entry_count] could help in a conditional but i'm not getting it to work. Does anyone have any ideas on how this could be completed? What I have working is below:

<div class="carousel-item [if entry_position equals=1]active[/if entry_position]">

<img class="img-fluid" src="[228 size=full]" />
<div class="carousel-caption d-none d-md-block">
[if 229]
<h5>[229]</h5>
[/if 229]
<p>
[if 232]
[232 truncate=10 more_text="..."]
[/if 232]
[if 230]
[230]
[/if 230]
[created-at]
</p>
</div>

</div>