It all depends on the layout of your website. If you are using a blog from blogger.com, I would say one on top one in middle and one at the bottom.
Best places to put your AdSense are top center or top left corner. Make sure your ads blend in with the colors of your site.
The best location for Google ads will vary from page to page. it will depend on the content of your page. These few questions to ask yourself will help you decide where to position your ads: 1. What is the user trying to accomplish by visiting my site? 2. What do they do when viewing a particular page? 3. Where is their attention likely to be focused? 4. How can I integrate ads into this area without getting in the users' way? 5. How can I keep the page looking clean, uncluttered and inviting?
The best position is always on the top or embedded in the content for more better position check the google adsense resource.