The first movie was based on the 1st Nanoha TV anime, while the second film was based on the second Nanoha TV anime, Magical Girl Lyrical Nanoha A’s.
According to a tweet from HMV’s anime retailing brand, this third Lyrical Nanoha movie will be having a brand new story which is not based on any previous material.
The magical girls are back with a new movie, and this third one will be titled Magical Girl Lyrical Nanoha Reflection.
In other words, the third movie might not be based on the Strikers TV anime.