I have a page that has three adsense placement. my problem is when i visit the page sometimes two of them will have the same ad!! can i make each one of them display a different ad?!! Thanks
I guess you are using image ads. That's out of our control. There is no way that we can show the ads of our choice. One thing you can try is, only if it's a problem with ads from few advertisers, block the ad by putting it in the competitive filter.
It probably means Google doesn't have a lot of ads that match your keywords. Try changing your keywords or content slightly to force different ads. Maybe broaden your keywords slightly. If your page is, for example, about "hairball control cat food", try making it more about "cat food" in general. Google will have more ads about "cat food."
Placing more ads doesn't mean you'll make more money. Just because you can place 3 blocks of ads doesn't mean you should.