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Since less ads are being shown, the ones that are shown should be the higher paying ones, hence a higher CPC.
I've actually only been running 2 for some time now on a newer portion of my site, but my CTR is abysmal so I'm going to throw in slot 3.
Personally, I feel that two Adsense blocks already give a page a cheap look. And most sites with three Adsense blocks look definitely cheap.
If this is not your concern, I'd say go for three ad blocks, check whether your eCPM improves, and make a final decision after the trial. That's the beauty of Adsense - you can just try anything without hurting anyone.
Personally, I feel that two Adsense blocks already give a page a cheap look. And most sites with three Adsense blocks look definitely cheap
I've not varied from 728 X 90 above the fold and a single line text ad at the bottom.
While AdSense is of course important to me it is definitely NOT the reason my sites exist. Never will be.