i was designing a new site where i was planning to integrate adsense into the html pages.i had a dreamweaver spry tabbed panels inserted in the html page.as we all know adsense allows only 3 ad units per page.
where i went wrong…..
so what i thought was i’ll integrate 2 ads on html page (outside spry tabbed panels) and the 3rd one inside the spry tabbed panels,like one ad for each tab.
so i’ll be displaying like 6 ads in the same page if i have 4 tabs,but still displaying only 3 ads on the page when considered the page as a whole.(because the other 3 ads would not be displayed as long as the tabs are open) 
what was the mistake?
i think google scans the ad units from the start of the page source and serves only the first 3 ad units.that is why the other 3 ad units were not displayed .
so i learnt my mistake after spending few hours .hope this post helps someone. 