Well I was recently informed that the manga and novels are fairly similar as far as the sequence of events goes. The anime diverts from the normal path and adds and takes out some major and minor events. Individually, aside from the story, they each have different elements. (character differences, drawing differences, feel differences, etc.)
Now, as for the third season. YES YES YES there will be a third season, a thousand times yes. Do not worry. The release date is what scares me. It will either air this fall in Japan, and then be subbed for all us non-japanese speakers. OR it will air next year (2012). No news yet on what their gonna do though. I'm really hoping that it comes out this year, as I can't bare the weight any longer. If you really dieing to see more shana, sadly the best you have is the anime. (unless you can read japanese) The english and fan translated novels are no where close to season 3. (Except for this excerpt
http://sites.google.com/site/asdigge...er-16-exerpt-a ) And the mangas sadly are at the same rate....So my advice would be to just wait until season 3.