If you are trying to insert an ad every 3 lines or 200 characters you need to modify $post['message'] in a plugin. There would be nothing to change or add to the templates.
There is a mod that will put an ad in the middle of the first post, I've seen it, it can be searched for.
Dear BirdOPrey, thank you so much for respone my post.
Yes i know the mod that will put an ad in the middle of the first post, but I dont want show ad in midle, i want show ad automatic in first 3 line or first 200 characters.
Can you give to me tutorial how to modify $post['message'], please!!