What i have found out is that Google will put the most expensive ad on the first set of google adsense code that they index, followed by the cheaper ones for the other two set. So u can optimize your page in a way that the more expensive ads appear on certain part of your page. Example you have one ad at the header and one at your navigation column. Your header ad have very little clicks but the clicks are high in value. Your navigation column get lots of clicks but the clicks are low in value. To get more expensive clicks on your navigation column, u have to make it the first block of adsense code in your html code. The trick is to add empty tables which will help arrange your adsense code. At least it worked for me..
Here it is. Can I display more than one set of Google ads on a web page? Currently, AdSense publishers may place up to three ad units on one web page. Our system will detect the multiple ad units and will display unique ads to each ad unit. This system is optimized for pages with highly targeted Google ads - some pages may not show ads in the second or third ad unit. In this case, the ad unit will show as a transparent box, or will contain any alternate ad or color specified in the ad code.
try seeing the first post of this page. you will see a line of text 'Ads by google xxxx xxxxx xxxxxx xxxxx' just underneath the first post. that is Adlink