#1 It depends

#2 Supposedly your highest paid ad will be the one that loads first.

My experience is that I reach a saturation point beyond which I think that AdWords advertisers begin to exhaust their collective daily budget. The effect looks to the publisher like "throttling." The result of fewer clicks is a higher average CPC rate, while the result of more clicks is a lower average CPC rate. The total revenue is about the same either way.

I've found that two large display-ad units (300x250 and/or 336x300) do just as well overall as three. What's more, using fewer units enhances user trust -- i.e., the page looks less like it was MFA (made for AdSense).

1. earn per click rises with less total earning. think about it, adsense is an auction system. so your second and third units ppc is almost likely to be lower.

2. average earn per click is typically higher at the top. but mind you depending on your users and setup, you may earn more or less clicks at the top.

best bet? based on my experience. don't worry about earn per click. EPC is a pointless game in most cases. worry about overall income instead. splatter all the ads, max out all the slots.

Make sure split into channels, figure out the best earner. Remove the other two slots and see how that affect your overall earning. EPC (per click) is a stage where you do keyword researches, not for adslot optimization. Worry about RPM instead, it's a much better metric.

The extensive testing I've done doesn't support the theory that the top ad earns more than other ads. Also, I've not seen a higher cpc for the first ad on a page if there is just 1 ad versus 2 or 3 ads. Like others said, put ads in the best places. That's about as strategic as you can get in July 2014.

My top ad earns best across a lot of sites, but that's just one datapoint. All you can do is test for yourself.