Final Season of Shakugan no Shana Due This Autumn [Update Aug 1]

According to Dengeki Bunko Magazine, Shakugan no Shana III will start this autumn. It will be the final season of Shana anime series.

Source: Scan of Dengeki Bunko Magazine

Update on August 1st
According to Dengeki Bootleg mail magazine, the final 22th volume of Shana novel will be released on October 10th.

Source: Dengeki Bootleg vol.119