displaying ads after various post positions

I have a client looking to display a variety of ads across multiple posts. What they want is a 468×60 ad to appear maybe three times max on their blog between posts. The blog will contain approximately 12-15 posts per page. They’re wanting the ads to be configurable as to where they appear… such as displaying maybe after only the 3rd post, 8th post and last post.

I looked into the various ad managers and cannot seem to find any that are configurable based on the post’s position on the page.

TIA