It has started airing in Japan. A third anime TV series titled Shakugan no Shana Final (灼眼のシャナIII -Final- Shakugan no Shana Fainaru?) began airing on October 8, 2011.

ASCII Media Works announced that the third season, titled Shakugan no Shana Final will air on October 7, 2011 in the July 2011 issue of their Dengeki Bunko Magazine.

Shakuga no shana Steam skin : Screen: http://dl.dropbox.com/u/9733425/shana%20skin%20beta.png
Download: http://dl.dropbox.com/u/9733425/Shana%20Skin%20Beta.zip

Will polish it up and release is in a week or two.
Credits to RiskyKen.

--Original post is in the comments.--