According to Animedia August issue, producer Nakayama Nobuhiro said the OVA series "Shakugan no Shana S
" is followed by the third season
We launched "Shana Project" with a view to ending the Shana anime series. Before moving on to the third season, we need to describe the backgrounds and the relationships between the characters more in detail. So the project began with the OVA series.
Source: Scan of Animedia
Related topic: New OVA Series of Shakugan no Shana Announced [Update]