I haven't seen anything set in stone that 1 unit makes more than 3 or anything of the sort. I think it's very dependent on the nature of the site's visitors (savvy vs. not-so-savvy), and how the ads are intergrated into the site.
Having more AdSense blocks definitely can lower your CPC, but could up your CTR... that is kind of up to your own stats to figure out which works better.
One thing that has worked well for me is setting each block on its own AdSense channel... I was able to see that one particular block (out of 3) performed dramatically better than the other two. At the time, that block was placed 2nd in the code of the page, and therefore displaying 2nd most profitable ads. Using CSS absolute positioning, I made it so that ad block was displayed first in the HTML, but still remained in the same place (2nd block on page) when the page was rendered in a browser. While it kept the same CTR, it bumped up my CPC about 35%.
