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To answer your original questions:
Ad units will "disappear" if there isn't enough ad inventory. Many people find that having only one ad unit pays the best because that eliminates the lowest-paying ads from your page. And it looks better to boot. So if you can't get all three to show, why not see how it goes with one?
Second, you'll do worse if you select image only, because you will be reducing the number of ads that COULD be shown on your page. If there aren't a lot of images ads in your area, and there actually may be none at all, you will get fewer ads showing.
Best approach is to allow for both text and image ads, and then the best-paying of both will show on your page.
Have you read some of the thread in the forum library? You could learn a lot.
In over two years with AdSense, I have seen an image ad on my site exactly once. Other people, in different subject areas, see them all the time.
For what it's worth, most people find that text ads have a higher click-through rate than image ads. Most surfers have learned to ignore image ads, but text ads, especially if well blended, get a second look. And from that second look, they may realize that they are interested.
In other words, don't worry if you don't see image ads. They may not do much for you anyway.