I have Win 7, NET 4.0.
I had the same issue win starting PF3...
I placed PF3 directory into ..\Users\MyAccount\AppData\Local\Ironclad Games
Then i copied these directories from original SOASE directory:
Mesh, Particle, PipelineEffects, TextureAnimations, Textures into PF3 directory (and did the same with Entrenchment directory).
Then i copied these files from SOASE directory:
With this PF3 runs correctly.