Running three of these ad units per page might be beneficial in certain cases, usually depending on what niche you're publishing in. Real Estate, for example, is rich with ads competing against each other, creating very high earnings per click (EPC) rates. Other niches might be much less competitive, and it would not make any sense to run several ad units.
When you run two or three ad units per page, a couple of dynamics start working against you...
- Introduction of lower paying ads - more ad units per page means more lesser-bidding ads are displaying. This will have the effect of dragging down your overall EPC.
- Competition between ad units - now each ad unit is competiting against each other for clicks, decreasing your chances of visitors clicking on the highest-bidding ads
Like I said before, it's not so bad if all of the ads were high-bidding ads. For example, if the top 12 ads for a specific keyword are bidding between $1.00 and $10.00 per click, then any click will produce great earnings for you. But if only the top two ads are bidding $1.00 or more, and the rest of the ads are bidding much lower at $0.06 to $0.12, then you'll end up a bunch of cheap ads competing against the two high-paying ads.
It's something you'll have to experiment with, and track closely. Make sure each ad unit has its own "channel", and track the combined earnings for all ad units. If you find the combined earnings decreasing as you introduce a second or third ad unit, then you'll know to remove the additional ad units.
Mixing Image Ads with Textual Ads
I don't advise running multiple ad units where one unit displays image ads, and another displays textual ads. In fact, I don't advise running AdSense image ads at all!
The reason is because image ads only display one ad per unit, and image ads tend to pay much poorly than textual ads. Making it worse is that some image ads display very compelling images designed to lure a lot of clicks. But these clicks pay poorly.
If you're going to run image ads, find another advertising network that pays purely on CPM, to monetize those visitors that don't click anything!