Zivush - 5:30 am on May 8, 2012 (gmt 0)
I never use three per page, and would certainly not have more, even if Google offered it to me.
I have never taken the offer. It looks to me spammy to put more than 2, sometimes 3. In some key pages 1 is the maximum I have.
1000 words per page is nothing, so I am surprised they offered this.
Correction: 1000 is the minimum on my site. The average is much more than 1000 however I can't specify exact # because the site contains 1200 articles and category pages.
The rep from Adsense told me that the reason there's 3 on their TOS is because they don't want publishers to push too many ads above the fold and because of their concerns on the ratio of ads per content on thin pages.
He even indicated this point, after talking to me, in front of many publishers during his speech at the stage.
My site is an articles site (long articles), so it seems reasonable.
I remember Adsense calling me once or twice "advising" me adding more ads.
BTW - The site is very close to get a premium publisher account.
All in all, # of ads is a decision that has to take revenues in one hand and user experience on the other (important) hand.
Again, never had the chance to get a rule of thumb on the optimal # of ads per page and that's why posted this thread.