Hello, Do you have the same perfomance if you have on the same webpage 1 or 3 ads? If you only have 1 ad you can have much better ctr and get more chances the higher paying sites to be displayed? While if you have 3 the higher paying sites are spread less chances to get clicked and also cpm falls down more due to less clicks with more impressions?
I have written an article about the subject giving my analysis on most of your question : http://adsense4money.com/AdSense/double-earnings-fewer-ads.php
The number of clicks didn't went down, but yes, i recieved much higher paying ads when i reduced to one ad unit. and obviously the ECPM improved.
It depends really on how much content you have on your pages. If you have just one ad at the top then you can have higher earnings per click, but if you have a lot of content, then there will be a higher chance of you probably getting a click if you have 2 - 3 ad units scattered accrossed the pages content. Overall, it's better to use less I think if you have not much content as it will look more professional to have less ads.