I wish they would explain what happened to Nanoha at that time IN DETAIL (like an ova or whatever, fuck they have 4 series but can't even make an ova out of the interesting stuff?)
The third movie is supposed to have an original plot and appears to focus on the time between A's and StrikerS; it wouldn't surprise me at all if they chose to explore that off-screen plot arc. That being said, it's a very negative way to end a movie, with Nanoha being severely wounded and facing life in a wheelchair.
They said it would not include any plotlines they already intruded. It wouldn't make for a great plot anyway. We already saw how she got injured. It wasn't a big boss battle or anything.