received below message twice from adsense (two different domain) and first time follow their instruction.

Our system analyzed 6067 pages across your domain,, and found only two Google ad units per page on 5951 of your pages. The AdSense program currently allows placing up to three ad units on each page, and we strongly suggest you do so.

Since a large portion of your site is not fully monetized, consider adding any of our top performing units (336x280, 300x250, 728x90 or 160x600) to pages currently showing only two ads in the placements our heatmap suggests:

We've found that by doing this, our publishers are able to place more than two ad unit sper page, maintain a good user experience, and achieve significant increases in revenue

After three day adding additional Ad unit (336x280), CTR: not much different but CPM make huge different from 0.7 to now min 1.2% it actually almost double my previous income ..
Click from new AD unit is 1.1 ctr which is pretty happy for me.