1) No (but you kind of can). By using more text boxes in your hub and splitting the text among the boxes, you allow more space in between for ads. This doesn't always get used, but it sometimes does, allowing you some minor control over ad placement.
2) When you create a new hub you can select none, low, medium, or high for your amount of ads, effectively allowing you to control the amount of ads on your website. You can also go back in and re-edit your hub to modify this later, should you decide you need more or less ads.
3) Adsense determines its ads from your content and from your keywords (also known as tags). Try including more tags on your hubpages and make them all specifically relevant to your content.
Many hubbers seem to think just getting traffic to their site is the secret to success, but I can tell you in my short time here that it's all about getting 'relevant' traffic.
Why? isn't more traffic better? I'll tell ya why. Those that visit your site have no clue how many visited before them. As such, if they are tricked to your site they are likely to bail quickly. Whereas, the traffic that arrives because they are looking for your content will stay longer - and if your content is good they will tell others, increasing the level of relevant traffic over time.
Not, if you do the above and no relevant image ads appear in two weeks, then (if you are advertising through Amazon) you might consider directly picking the products you advertise. If you are only using adsense, then it's 'rewrite or cope'.
I hope these answers help you. :)