as long as the content is clearly visible without scolling the page, you're fine. But if you feel a little uneasy about it, I suggest you save the last block for the end of your content. Use a 336x280 right after your content, that will also be clicked regularly and you'll have only 2 blocks at the top.
Three ad groups at the top - it is toooooo much! You''l probably will not be banned - but its ugly - no one will click! Try to pur your adds in the all "hot areas" - 1 on top, 1 after the the text, and 1 more at the right side of the pade - its classic and classic is always works!
And you can put also an adsense for search on your site if you don't have. For me is working very good and is located in the right side of header.