In the AdSense setup page it shows "Ad Unit" and "Link Unit" as two distinct types of ads. Under "Google AdSense Program Policies" it says: Up to three ad units may be displayed on each Web site page. A single link unit may be placed on each Web site page, in addition to the ad units and search boxes specified above. Link units are considered to be 'Google ads' for purposes of these program policies. The first one says "three ad units" not "three Google ads", but I've seen alot of comments saying that only three ads per page is allowed. If it is that there is a maximum of three Google ads per page, I think they should change the first point to "Up to three 'Google ads' may be displayed on each web site page." Is your interpretation of this that three ad units plus one link unit can be on a page? Or, Is your interpretation of this that two ad units plus one link unit can be on a page, or three ad units and no link units? Hopefully I'm making some sense.
I beleive their rule is that you can have 3 ad units and 2 link units. If you try to do more than that they simply wont display all of the ads - they will only diplay the maximum.