an ad every three posts

Have you checked out angsuman’s plugin ?
https://blog.taragana.com/index.php/archive/wordpress-plugin-adrotator-rotate-your-ads-including-adsense-dynamically/

I use CG-Inbetween, nice new plugin from David Chait. You may set the ad just about anywhere (A number 1-n | all | last | single). It also allows you to echo info, perform a function, or include a file).

I looked at it, but it doesn’t seem to have the option of showing up after a certain number of posts – just after every post??

where can i get CG-Inbetween ?

https://www.chait.net/index.php?p=310

David is always looking to enhance his plugins and I have yet to have him not respond to any reasonable request. You might ask him to include an option for “after every X posts” – I am assuming it would not be very difficult.

For now it would be easy to set 1, 3, 6, 9, 12 (surely you don’t have much more on any one page).

sorry beel I was referring to the plugin podz mentioned = )

the one you mentioned looks good, so Ill give it a try