Question: I have a zone that links to 1000 banners.

I want to use the invocation code in the top of the page, as well as the center, and bottom.

How do I have this code in the page 3 times, but ensure that it will not display the same and twice at the same time. Meaning if banner appears at the top, it should not appear anywhere else on page during that particular load.

 
Ans:  Solution is available in our OpenX. While generating invocation code you can see these options "Don't show the banner again on the same page" .you can use this function for your questions
Also you can restrict the banners from the same campaign also by using this option "Don't show a banner from the same campaign again on the same page".